Login| Sign Up| Help| Contact|

Patent Searching and Data


Title:
STEAM BOILER ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2019/231325
Kind Code:
A1
Abstract:
The present invention relates to a steam boiler assembly (1), configured to heat water in order to generate steam, comprising at least one boiler (20, 30), a fuel heater (23), an electric heater (35, 36), a feed water inlet (11), a steam outlet (12), and a controller (50), wherein the controller (50) is configured to selectively activate or deactivate the fuel heater (23) and/or the electric heater (35, 36) to bring and keep the water in the at least one boiler (20, 30) at saturated conditions. The steam boiler assembly (1) further comprises a water-tube boiler (20), which comprises the fuel heater (23), an electrode boiler (30), which comprises the electric heater (35, 36), and a steam drum (10), which is configured to hold water and steam, wherein the feed water inlet (11) and the steam outlet (12) are fluidly connected to the steam drum (10). The water-tube boiler (20) and the electrode boiler (30) each comprise an inlet (21, 31) and an outlet (22, 32), which are fluidly connected to the steam drum (10). The inlet (21) of the water-tube boiler (20) is fluidly connected to the inlet (31) of the electrode boiler (30) and the interconnected inlets (21, 31) are configured to allow a flow of water from an activated one of the boilers (20, 30) to a deactivated one of the boilers (30, 20) in order to keep the water in the deactivated boiler at saturated conditions.

Inventors:
BRAMER ALBERTUS BERNARDUS (NL)
Application Number:
NL2019/050315
Publication Date:
December 05, 2019
Filing Date:
May 31, 2019
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
STORK THERMEQ B V (NL)
International Classes:
F22B1/18; F22B1/30; F22B33/00; F22B35/00; F22B37/14; F22B37/22
Foreign References:
DE102014201406B32014-12-24
US4745758A1988-05-24
US20040093124A12004-05-13
CH159498A1933-01-15
Attorney, Agent or Firm:
RIEMENS, R.H. (NL)
Download PDF:
Claims:
CLAIMS

1. Steam boiler assembly (1), configured to heat water in order to generate steam,

comprising:

at least one boiler (20,30), comprising walls that define an interior that is configured to hold the water;

a fuel heater (23), configured to heat the water in the at least one boiler (20,30) upon the burning of fuel,

an electric heater (35,36), configured to heat the water in the at least one boiler

(20.30) with electric energy,

a feed water inlet (11), configured to feed water into the steam boiler assembly

(1),

a steam outlet (12), configured to discharge steam from the steam boiler assembly (1), and

a controller (50), configured to control the fuel heater (23) and the electric heater (35,36),

characterized in that the at least one boiler (20, 30) comprises:

a water-tube boiler (20), which comprises the fuel heater (23),

an electrode boiler (30), which comprises the electric heater (35,36), and in that the steam boiler assembly (1) further comprises:

a steam drum (10), comprising walls that define an interior that is configured to hold water and steam,

wherein the controller (50) is configured to selectively activate or deactivate the fuel heater (23) and/or the electric heater (35,36) to bring and keep the water in the at least one boiler (20,30) at saturated conditions,

wherein the feed water inlet (11) and the steam outlet (12) are fluidly connected to the steam drum (10),

wherein the water-tube boiler (20) and the electrode boiler (30) each comprise an inlet

(21.31), which is fluidly connected to the steam drum (10) and which is adapted to allow the entrance of water from the steam drum (10) into the respective boiler (20,30), wherein the water-tube boiler (20) and the electrode boiler (30) each comprise an outlet

(22.32), which is fluidly connected to the steam drum (10) and which is adapted to allow the exit of steam from the respective boiler (20,30) into the steam drum (10), and wherein the inlet (21) of the water-tube boiler (20) is fluidly connected to the inlet (31) of the electrode boiler (30) and wherein the interconnected inlets (21 , 31) are configured to allow a flow of water from an activated one of the water-tube boiler (20) or the electrode boiler (30) to a deactivated one of the water-tube boiler (20) or the electrode boiler (30) in order to keep the water in the deactivated boiler at saturated conditions.

2. Steam boiler assembly (1) according to claim 1 , further comprising a return conduit (42), which is fluidly connected to the water-tube boiler (20) and the electrode boiler (30) and which is configured to allow a return flow of water from the deactivated boiler to the activated boiler.

3. Steam boiler assembly (1) according to claim 1 or 2, comprising at least one valve (41), which is respectively arranged in at least one of the inlets (21 ,31) and which is configured to guide the flow of water in the inlets (21 ,31), wherein the controller (50) is configured to control the at least one valve (41) to guide the flow of water in inlets (21 ,31).

4. Steam boiler assembly (1) according to claim 3, wherein the controller (50) is

configured to control the at least one valve (41) to guide a flow of water through the inlets (21 ,31) from the water-tube boiler (20) to the electrode boiler (30) when the water-tube boiler (20) is activated, in order to keep the water in the electrode boiler (30) at saturated conditions.

5. Steam boiler assembly (1) according to claim 3 or 4, wherein the controller (50) is

configured to control the at least one valve (41) to guide a flow of water through the inlets (21 ,31) from the electrode boiler (30) to the water-tube boiler (20) when the electrode boiler (30) is activated, in order to keep the water in the water-tube boiler (20) at saturated conditions.

6. Steam boiler assembly (1) according to any of the preceding claims, wherein the

controller (50) is configured to selectively activate or deactivate the fuel heater (23) and/or the electric heater (35,36) on the basis of a price per unit of energy for the fuel (Pf) and for the electricity (Pe).

7. Steam boiler assembly (1) according to claim 6, wherein the controller (50) is

configured to:

activate the fuel heater (23) to heat the water in the water-tube boiler (20) when the price per unit of energy is lower for the fuel (Pf) than for the electricity (Pe), and to activate the electric heater (35,36) to heat the water in the electrode boiler (30) when the price per unit of energy is lower for the electricity (Pe) than for the fuel (Pf).

8. Steam boiler assembly (1) according to any of the preceding claims, further comprising a steam turbine (61), which is fluidly connected to the steam outlet (12) and, when activated, configured to transform thermal energy from a portion of the generated steam into electric energy.

9. Steam boiler assembly (1) according to claim 8, wherein the controller (50) is further configured to selectively activate or deactivate the steam turbine (61).

10. Steam boiler assembly (1) according to any of the preceding claims, wherein the

electric heater (35,36) comprises at least two electrodes (36), which are arranged at least partially in the interior (34) of the electrode boiler (30) and wherein the electric heater (35,36) is configured to induce an electric current between the electrodes (35) in order to heat the water in the electric boiler (30).

11. Method (100) for the controlling of a steam boiler assembly (1) according to any of the preceding claims, comprising the steps of:

obtaining (110) a price per unit of energy for fuel (Pf),

obtaining (120) a price per unit of energy for electricity (Pe),

determining (130) which of the prices per unit of energy (Pf, Pe) is the lowest, and controlling the steam boiler assembly (1) to:

o activate the fuel heater (12) when the price per unit of energy is lower for the fuel (Pf) than for the electricity (Pe); and to

o activate the electric heater (35,36) when the price per unit of energy is lower for the electricity (Pe) than for the fuel (Pf).

12. Method (100) according to claim 11 , further comprising the step of:

determining (160) an amount of steam (Qs) that is required to be generated by the steam boiler assembly (1),

wherein the step of controlling further comprises the controlling of the steam boiler assembly (1) to:

o activate the fuel heater (23) and the electric heater (35,36) when the required amount of steam (Qs) is larger than a maximum steam production capacity of the water-tube boiler (20) or the electric boiler (30); and to o deactivate the fuel heater (23) and the electric heater (35,36) when no steam is required.

13. Method (100) according to claim 11 or 12, further comprising the steps of:

obtaining (140) a pool reward (PR) for accumulating and/or dissipating electric energy,

correcting (150) the price per unit of energy for the electricity (Pe) with the pool reward (PR) to obtain a corrected price per unit of energy for the electricity (Pe’), wherein the step of determining (130) which of the prices per unit of energy is the lowest comprises the determining of the lowest of the price per unit of energy for the fuel (Pf) and the corrected price per unit of energy for the electricity (Pe’). 14. Method (100) according to claim 13, wherein the steam boiler assembly (1) is a steam boiler assembly (1) according to claim 8 or 9, wherein the method (100) comprises the step of activating the steam turbine (61) when the pool reward (PR) for dissipating electric energy is larger than the price per unit of energy for the fuel (Pf). 15. Method to generate steam, for example for an industrial process, comprising the steps of:

controlling, with a method (100) according to any of the claims 11 - 14, a steam boiler assembly (1) according to any of the claims 1 - 10,

heating water with a water-tube boiler (20) and/or an electric boiler (30) of the steam boiler assembly (1) in order to generate steam, and

repeating the steps of the controlling the steam boiler assembly (1) and the heating of the water.

Description:
Title: Steam boiler assembly

The present invention relates to a steam boiler assembly. The invention further relates to a method for the controlling of a steam boiler assembly and to a method to generate steam.

As a result of the increasing amount of sustainable resources for electricity, such as wind turbines, solar panels or hydro power, it occurs that the actual production of electricity may no longer be controlled as accurately as before, when the majority of electricity was generated by means of conventional power stations. The fluctuations in power supply result in surpluses or lacks of electricity, which give, in turn, rise to fluctuations in the price of the electricity.

From Swiss patent CH 159498, a fire tube steam boiler is known, which is configured to heat water by burning fossil or non-fossil fuels or by means of electric heating. The boiler comprises a vessel, which is filled with water and which comprises a plurality of fire tubes, extending through the water. The boiler further comprises a heater, which is configured to burn fossil fuel in order to obtain hot gasses that flow through the fire tubes to heat the water in the vessel.

The known boiler furthermore comprises resistive heating elements, which are also provided in tubes through the water. The temperature of the heating elements increases when an electric current is guided through them and the increased temperature of the heating elements causes the temperature of the water to increase as well. The heating with fossil fuels and electricity may be done simultaneously, in case of sudden load peaks, or subsequently, depending on the lower rate or higher rate of the prices for the electricity.

The known steam boiler does, however, have the disadvantage that it is not able to operate at high pressure. The known steam boiler is provided as a fire tube boiler, which generally has a maximum working pressure of approximately 25 bars, governed by the strength of the boiler.

The known boiler does, furthermore, not allow the electrical heating to be retrofitted in an existing fuel-heated steam boiler. In case it would be desired to convert an existing fuel- heated steam boiler into the known steam boiler with electric heating, it is needed to provide additional tubes in the boiler for these resistive heating elements. The provision of such additional tubes requires significant structural modifications of the boiler, which is highly disadvantageous during retrofitting, which preferably consists of minor adaptions.

The known steam boiler also has a relatively large volume of water, which means that the relevant time intervals, for example the time between a cold start and a working conditions steam pressure, are relatively long. Another time interval may be the downtime between switching from fuel heating to electric heating or vice versa, which is significantly large as well, due to the thermal inertia of the large volume of water.

It is an object of the present invention to overcome the disadvantages of the known steam boiler, or at least to provide an alternative steam boiler.

The present invention provides a steam boiler assembly, configured to heat water in order to generate steam, comprising:

at least one boiler, comprising walls that define an interior that is configured to hold the water;

a fuel heater, configured to heat the water in the at least one boiler upon the burning of fuel,

an electric heater, configured to heat the water in the at least one boiler with electric energy,

a feed water inlet, configured to feed water into the steam boiler assembly

a steam outlet, configured to discharge steam from the steam boiler assembly, and a controller, configured to control the fuel heater and the electric heater,

characterized in that the steam boiler assembly further comprises:

a water-tube boiler, which comprises the fuel heater,

an electrode boiler, which comprises the electric heater, and

a steam drum, comprising walls that define an interior that is configured to hold water and steam,

wherein the controller is configured to selectively activate or deactivate the fuel heater and/or the electric heater to bring and keep the water in the at least one boiler at saturated conditions, wherein the feed water inlet and the steam outlet are fluidly connected to the steam drum, wherein the water-tube boiler and the electrode boiler each comprise an inlet, which is fluidly connected to the steam drum and which is adapted to allow the entrance of water from the steam drum into the respective boiler, wherein the water-tube boiler and the electrode boiler each comprise an outlet, which is fluidly connected to the steam drum and which is adapted to allow the exit of steam from the respective boiler into the steam drum, and wherein the inlet of the water-tube boiler is fluidly connected to the inlet of the electrode boiler and wherein the interconnected inlets are configured to allow a flow of water from an activated one of the water-tube boiler or the electrode boiler to a deactivated one of the water-tube boiler or the electrode boiler in order to keep the water in the deactivated boiler at saturated conditions.

The steam boiler assembly according to the present invention comprises a water-tube boiler and an electrode boiler, which are provided separate from each other, e.g. not forming an integrated unit with a single body of water. The water-tube boiler and the electrode boiler are arranged in a parallel configuration and are each configured to operate in parallel and to heat a respective body of water, e.g. to generate the steam in parallel. This differs from the known boiler, in which the fuel heater and the electric heater are incorporated to heat the same body of water. The separate water-tube boiler and electrode boiler provide the advantage that, in order to construct the steam boiler assembly, existing water-tube boilers and electrode boilers may be used, which may be fluidly connected to form the steam boiler assembly. This opposed to the incorporated water- tube boiler and electrode boiler from the known steam boiler, which requires adaptions to the setup of each of the water-tube boiler or the electrode boiler, for example to allow for the introduction of electric heating elements.

The fuel-heated boiler in the steam boiler assembly according to the invention is a water-tube boiler, which comprises at least one fluid passage, configured to guide a fluid flow. The fluid passage is arranged in a flue gas area of the fuel heater, typically the combustion chamber, or chimney. The flue gasses from the burned fuel pass along the fluid passage, during which heat is transferred from the flue gasses onto the water in the fluid passage.

In the fire tube boiler from the known steam boiler, the flue gasses are led through gas passages, which extend through a body of water. The water-tube boiler in the steam boiler assembly according to the present invention is, compared to the fire tube boiler in the prior art steam boiler, generally capable of withstanding larger pressures. This is caused by the fact that the fluid passages have a relatively high surface to volume ratio, when compared to the water body of the know fire-tube boilers. The larger pressures allow that the steam boiler assembly according to the invention may operate more efficient, compared to the fire tube boiler from the prior art, a typical working steam pressure of a water-tube boiler may run up to 160 bars.

A further advantage of the water-tube boiler, in view of a fire-tube boiler, lies in the fact that the required amount of water in the boiler may be significantly less for a similar steam output. This has, first of all, the advantage that the water-tube boiler may be more compact than the fire-tube boiler, for a similar steam output. The smaller water-tube boiler furthermore provides that the thermal inertia is reduced compared to the fire-tube boiler, which means that the water-tube boiler can react more rapidly to changes in temperature, for example reducing the required reaction time when the boiler were to be activated from cold.

The fuel heater in the water-tube boiler is preferably a gas-type heater, which is configured to generate heat upon the burning of gaseous fuels. The advantage of burning gas, in particular compared to burning solid fossil fuels, such as coal or biomass, is that the fuel heater may be activated or deactivated rapidly and that the amount of generated heat may be easily adjusted by adjusting a flow of fuel towards the heater. The water-tube boiler is, in an embodiment of the steam boiler assembly, of the concurrent type, wherein, during use, a flow direction of flues gasses is aligned parallel to a direction of the fluid flow through the at least one fluid passage.

In an alternative embodiment, the water-tube boiler is of the counter flow or cross flow type, wherein, during use, a flow direction of flues gasses is aligned antiparallel, e.g. opposite to a direction of the fluid flow through the at least one fluid passage.

The electric heater is, in the steam boiler assembly according to the present invention, incorporated in an electrode boiler. The electric heater comprises at least two electrodes, which are arranged in the interior of the electrode boiler and which are in contact with the water in the interior. During use of the electrode boiler, an AC voltage is applied over the electrodes and a resulting AC current is induced through the water, provided that the electric conductivity of the water is sufficient. The alternating electric current thereby causes the water to heat in order to generate the steam.

It is noted that the conductivity of the water in the electrode boiler, and thus the steam boiler assembly, may be increased by addition of conductive substances, like salts. It has, however, been found that the water quality that is used in conventional fuel-heated steam boilers, such as fire tube or water-tube boilers, is already sufficiently large to be used in an electrode boiler as well.

The electric heater in the known steam boiler comprises resistive elements, which are configured to increase in temperature when an electric current is guided through them. This means that the heating is indirect, because the heat has to be transferred from the resistive elements onto the water. In the electrode boiler according to the present invention, the heat is generated in the water itself and does not need to be transferred. This decreases a reaction time of the boiler, and allows the electrode boiler to be switched more rapidly from cold into activated, working conditions.

An additional advantage of the electrode boiler, compared to an electric boiler with resistive elements, is that it does not require a large interfacial area between the heated resistive elements and the water, which means that the electrode boiler may be dimensioned smaller than an electric boiler with resistive elements, while having a similar steam output.

The smaller dimensions reduce, similar as for the water-tube boiler, the thermal inertia of the electrode boiler, compared to the boiler with resistive elements.

The inlets of the water-tube boiler and the electrode boiler are each connected to the steam drum, in order receive liquid water from the steam drum. The water-tube boiler may comprise an inlet, which is fluidly connected to the steam drum and which is adapted to allow the entrance of water from the steam drum. The electrode boiler may comprise an inlet, which is fluidly connected to the steam drum and which is adapted to allow the entrance of water from the steam drum. This parallel configuration may provide that the water-tube boiler receives water directly from the steam drum and that the electrode boiler receives water directly from the steam drum. The water-tube boiler and the electrode boiler are thereby configured to operate parallel to each other.

This parallel configuration differs configuration in which the boilers are arranged in series, in which an inlet of one boiler is connected to an outlet of another boiler. Such a serial configuration would have the result that the one boiler receives water from the other boiler, instead of directly from the steam drum.

The water is, during use of the steam boiler assembly, inserted into the respective boiler to be heated. The steam boiler assembly further comprises a feed water inlet, which is fluidly connected to the steam drum and which is, during use of the steam boiler assembly, configured to guide a flow of feed water that is fed into the steam boiler assembly. The feed water thereby compensates for the steam that is discharged from the steam boiler assembly, in order to maintain a sufficient amount of water in the steam boiler assembly.

The outlets of the water-tube boiler and the electrode boiler are each connected to the steam drum as well, in order to discharge the generated steam from the boilers into the steam drum. The water-tube boiler may comprise an outlet, which is fluidly connected to the steam drum and which is adapted to discharge the generated steam into the steam drum. The electrode boiler may comprise an outlet, which is fluidly connected to the steam drum and which is adapted to discharge the generated steam into the steam drum.

This parallel configuration may provide that the water-tube boiler discharges steam directly into the steam drum and that the electrode boiler discharges steam directly into the steam drum. The water-tube boiler and the electrode boiler are thereby configured to operate parallel to each other.

This parallel configuration differs configuration in which the boilers are arranged in series, in which an outlet of one boiler is connected to an inlet of another boiler. Such a serial configuration would have the result that the one boiler discharges steam into the other boiler, instead of discharging directly into the steam drum.

The steam boiler assembly further comprises a steam outlet, which is fluidly connected to the steam drum and through which the steam boiler assembly is configured to discharge the generated steam.

The steam boiler assembly further comprises a controller, which may be electrically connected to the fuel heater of the water-tube boiler and to the electric heater of the electrode boiler. The controller is configured to selectively activate and/or deactivate the water-tube boiler and the electrode boiler. When a respective boiler is activated during use of the steam boiler assembly, heat is produced by the respective heater and the water in the respective boiler is heated to generate steam. When a respective boiler is deactivated during use of the steam boiler assembly, the respective heater is turned off.

The controller may furthermore be configured to activate the water-tube boiler and the electrode boiler such, that the amount of steam that is generated by the steam boiler assembly substantially corresponds to an amount of steam that is desired from the steam boiler assembly. The controller may thereby, for example, activate only one of the water-tube boiler or the electrode boiler, or may activate both boilers, but at a fraction of their maximum productions capacities.

In the steam boiler assembly, the inlet of the water-tube boiler is fluidly connected to the inlet of the electrode boiler. Both inlets may thereby a branch of a common water inlet, extending from the steam drum. The interconnected inlets of both boilers may thereby bypass the steam drum during the provision of a flow between the water-tube boiler and the electrode boiler. The interconnected inlets are configured to guide a fluid flow from an activated one of the water-tube boiler or the electrode boiler to a deactivated one of the boilers.

When a boiler is deactivated, the water in the respective boiler is not heated and may cool down. In case it is desired to switch a boiler from being deactivated to being activated, the water in the deactivated boiler must first be heated to its boiling point, e.g. to saturated conditions, before steam can be generated. This heating of the water towards the saturated conditions takes time, and therefore increases the required time for the switching between boilers.

The fluid flow through the interconnected inlets is directed from the activated boiler to the deactivated boiler, in order to prevent the water in the deactivated boiler from cooling down. The heated water from the activated boiler is fed into the deactivated boiler, which substantially prevents the temperature in the deactivated boiler from decreasing, thereby maintaining saturated conditions.

Preferably, the water-tube boiler may be primarily activated during use of the steam boiler assembly and may form the primary boiler of the steam boiler assembly. The electrode boiler is deactivated and may form a backup for the water-tube boiler or may be activated when the steam demand from the steam boiler assembly is larger than the maximum capacity of the water-tube boiler. During the normal use of the steam boiler assembly, the inlet of the electrode boiler guides a fluid flow from the water-tube boiler towards the electrode boiler, in order to prevent the electrode boiler from cooling down and to reduce the required switching time when the electrode boiler were to be activated.

In other cases, for example when a surplus of electricity would be present, the electrode boiler may be primarily active, whereas the water-tube boiler may then be deactivated during normal use. In an embodiment, the steam boiler assembly comprises a return conduit, which is fluidly connected to the water-tube boiler and the electrode boiler and which is configured to allow a return flow of water from the deactivated boiler to the activated boiler. The return flow provides that the water that is fed into the deactivated boiler may be returned, without having to return water through the outlet of the deactivated boiler, which means that the steam drum is bypassed.

The return conduit may for example extend between the electrode boiler and the inlet of the water-tube boiler. This means that, during the normal use of the steam boiler assembly, the return flow is directed from the electrode boiler into the inlet of the water-tube boiler. The water that is fed into the electrode boiler through its inlet is then discharged via the return conduit and fed back in the water-tube boiler. The water does not flow through the steam drum, which means that the conditions of the water in the steam drum are not influenced by the bypass flow of water between the water-tube boiler and the electrode boiler.

In an embodiment, the steam boiler assembly comprises at least one valve, which is respectively arranged in at least one of the inlets and which is configured to guide the flow of water in the inlets, wherein the controller is configured to control the at least one valve to guide the flow of water in inlets.

The at least one valve is configured to guide the flow through the inlets in a certain direction, wherein this direction is governed by the controller. When the water-tube boiler is activated, the controller controls the at least one valve to guide the flow through the inlets towards the electrode boiler. When, however, the electrode boiler is activated, the at least one valve is controlled by the controller to guide the flow through the inlets in the opposite direction, e.g. towards the water-tube boiler.

In a further embodiment, the controller is therefore configured to control the at least one valve to guide a flow of water through the inlets to the electrode boiler when the water- tube boiler is activated, in order to keep the water in the electrode boiler at saturated conditions.

In an alternative or additional embodiment, the controller is configured to control the at least one valve to guide a flow of water through the inlets to the water-tube boiler when the electrode boiler is activated, in order to keep the water in the water-tube boiler at saturated conditions.

In an embodiment of the steam boiler assembly, the controller is configured to selectively activate or deactivate the fuel heater and/or the electric heater on the basis of a price per unit of energy for the fuel and for the electricity. The price per unit of energy thereby determines which of the boilers is activated and which of the boilers is deactivated. The controller controls the boilers to generate steam at the lowest possible cost, thereby choosing between burning fuel or using electricity to heat the water and to generate the steam. Fast switching times due to maintaining the respective deactivated boiler under saturated conditions via the leaking flow from the activated boiler and the type of heaters used in the invention allow to minimize the costs of steam generation in time compared to slow switching and non-stand-by boilers.

The prices per unit of energy are defined as the costs for a certain predefined amount of fuel, comprising a certain amount of potential thermal energy being incorporated in the unburned fuel, or for a predetermined amount of electric energy. The price per unit energy may be corrected for the efficiency of the respective boiler and thereby becomes a measure of the price per unit of volume of steam being generated by the water-tube boiler or the electrode boiler. By activating the respective boiler with the lowest price per unit of energy, the steam can be generated the least expensive.

In a further embodiment of the steam boiler assembly, the controller is configured to activate the fuel heater to heat the water in the water-tube boiler when the price per unit of energy is lower for the fuel than for the electricity. In this situation, the price per unit of volume of steam is lower for the water-tube boiler than for the electrode boiler, which means that steam being generated by the water-tube boiler is the least expensive.

In this embodiment of the steam boiler assembly, the controller is further configured to activate the electric heater to heat the water in the electrode boiler when the price per unit of energy is lower for the electricity than for the fuel. In this situation, the price per unit of volume of steam is lower for the electrode boiler than for the water-tube boiler, which means that steam being generated by the electrode boiler is the least expensive.

In an embodiment, the steam boiler assembly further comprises a steam turbine, which is fluidly connected to the steam outlet and, when activated, configured to transform thermal energy from a portion of the generated steam into electric energy. The steam turbine is configured to receive steam from the steam outlet, being discharged from the steam drum, wherein the steam is led across turbine blades of the steam turbine. The turbine blades are rotated under the influence of the steam and the rotational energy from the turbine may be transformed into electric energy, which may be fed back to an electricity power distribution network.

This steam turbine may be advantageous when the price per unit of energy of the fuel is lower, preferably significantly lower, than the price per unit of energy for the electricity. In this situation, the water-tube boiler is activated to generate steam, whereas the electrode boiler is deactivated. When the amount of generated steam by the steam boiler assembly is larger than a required amount of the steam, a remaining portion of the steam may be used to generate electricity by means of the steam turbine, wherein the generated electricity may be used at the plant itself or may be fed back to the electricity power distribution network. In this situation, the generated electricity may be sold for a price of energy that is higher than the price per unit of energy for the fuel, meaning that a net profit may be achieved by selling the electricity, while the steam boiler assembly may still keep on generating steam.

In a further embodiment, the controller is further configured to selectively activate or deactivate the steam turbine. The controller is thereby configured to determine which of the prices per unit of energy is the lowest and may, on the basis of these prices, select which of the boilers becomes activated to generate the steam and, when the water-tube boiler is activated, whether the steam turbine is activated as well in order to generate electricity.

The present invention further provides a method for the controlling of a steam boiler assembly according to the invention, comprising the steps of:

obtaining a price per unit of energy for fuel,

obtaining a price per unit of energy for electricity,

determining which of the prices per unit of energy is the lowest, and

controlling the steam boiler assembly to:

o activate the fuel heater when the price per unit of energy is lower for the fuel than for the electricity; and to

o activate the electric heater when the price per unit of energy is lower for the electricity than for the fuel.

With the method according to the invention, as explained above, the steam boiler assembly may be controlled to generate the steam at the lowest possible price. When the price for the fuel is relatively low compared to that of the electricity, the cost price per unit of volume of steam is lower when the steam were to be generated with the water-tube boiler as compared to when it were generated by means of the electrode boiler. In this situation, the water-tube boiler of the steam boiler assembly may be controlled to burn the fuel in order to generate the steam.

When, however, the price for the electricity is relatively low compared to that of the fuel, the cost price per unit of volume steam is lower when the steam were to be generated with the electrode boiler as compared to when it were generated by means of the water-tube boiler. In this situation, the electrode boiler of the steam boiler assembly may be controlled to generate the steam.

The method according to the invention is preferably carried out by means of a controller of the steam boiler assembly, which is configured to carry out the steps of the obtaining of the prices per unit of energy, the step of the determining which of the prices per unit of energy is the lowest and the step of the controlling of the steam boiler assembly to activate a respective one of the water-tube boiler or the electrode boiler. In an embodiment, the method further comprises the step of determining an amount of steam that is required to be generated by the steam boiler assembly, wherein the step of controlling further comprises the controlling of the steam boiler assembly to:

o activate the fuel heater and the electric heater when the required amount of steam is larger than a maximum steam production capacity of the water-tube boiler or the electric boiler; and to

o deactivate the fuel heater and the electric heater when no steam is required.

In this embodiment, the controlling of the water-tube boiler and the electrode boiler is not only done on the basis of the prices per unit of energy for the fuel and the electricity, but also on the basis of an amount of steam that is required from the steam boiler assembly. In case the required amount of steam is larger than the maximum steam production capacity of the water-tube boiler or the electrode boiler, this sole activated boiler is not able to generate the required amount of steam. In this situation, the other boiler of the steam boiler assembly is activated as well, such that the steam is generated simultaneously by both boilers.

The water-tube boiler and the electrode boiler are preferably controlled such, that the one boiler for which the respective price per unit of energy is the lowest, will operate at its maximum production capacity, whereas the other boiler, for which the respective price per unit of energy is the highest, will generate steam at the lowest possible production rate, while still being able to comply with the required amount of steam.

In case, for example, the price per unit of energy is the lowest for the fuel, the water- tube boiler is activated to generate the steam. When the required amount of steam is larger than the maximum steam production capacity of the water-tube boiler, the electrode boiler is activated to generate an amount of steam that is just sufficient to bridge the gap between the maximum steam production capacity of the water-tube boiler and the required amount of steam.

In case, however, no steam is required from the steam boiler assembly, for example during downtime of the plant to which the steam is normally delivered, both the water-tube boiler and the electrode boiler may be deactivated. In this situation, the water is not heated and no steam is delivered to the plant.

In an additional embodiment of the steam boiler assembly, when no steam is required from the steam boiler assembly, the respective boiler for which the price per unit of energy is the lowest is activated to generate an amount of steam that is sufficient to keep the water in both boilers at the saturated conditions. In this situation, the steam boiler assembly does not deliver any steam through the steam outlet, but the amount of generated steam is set to be just sufficient to prevent the water in the interior of the water-tube boiler and the electrode boiler from cooling down.

In an alternative or additional embodiment, the method further comprises the steps of: obtaining a pool reward for accumulating and/or dissipating electric energy, correcting the price per unit of energy for the electricity with the pool reward to obtain a corrected price per unit of energy for the electricity,

wherein the step of determining which of the prices per unit of energy is the lowest comprises the determining of the lowest of the price per unit of energy for the fuel and the corrected price per unit of energy for the electricity.

With the recent developments in renewable methods to generate electricity, such as solar energy or wind energy, the availability of electricity may not be as constant as it used to be, when the electricity was almost only generated in conventional power stations. These renewable sources of energy may give rise to fluctuations in the loading of the electricity power distribution network, because a solar panel may not generate as much electricity when the sun is covered by a cloud or because a wind turbine can generate more electricity during high winds as compared to calm weather.

In certain jurisdictions, the electricity distribution network operator has introduced pool rewards to companies that are capable of compensating the fluctuations in the loading of the electricity power distribution network. The height of the pool reward may be dependent on a reaction time, e.g. a time for receiving and processing a signal to accumulate or dissipate electricity, and a maximum amount of compensation, e.g. the amount of electricity that can be accumulated (consumed) from the distribution network or dissipated (generated) into the distribution network within the reaction time. The pool reward is the largest for companies that are able to both accumulate and dissipate a large amount of electricity at the shortest possible reaction time. The pool reward may vary over time, but is generally significantly high when the electricity power distribution network suffers from an unallowable peak or an unallowable valley.

The steam boiler assembly is configured to accumulate electricity by generating steam with the electrode boiler of the steam boiler assembly, thereby consuming electricity. The method for the controlling of the steam boiler assembly comprises, in the present embodiment, the step of the obtaining of the pool reward. The pool reward is considered to be a discount on the price per unit of energy for the electricity. Hence, the sum of the pool reward for accumulating electricity and of the price per unit of energy for the electricity is calculated to be a corrected price for the electricity. It is noted that this corrected price may vary over time, as a result of the varying pool reward and the varying price per unit of energy for the electricity.

In this embodiment of the method, the corrected price per unit of energy for the electricity is compared with the price per unit of energy for the fuel. In case the corrected price is found to be the lowest, the electrode boiler of the steam boiler assembly is activated to generate the steam that is required from the steam boiler assembly, thereby consuming electricity. In case it is found that, despite the pool reward, the price per unit of energy is the lowest for the fuel, the electrode boiler is deactivated and the water-tube boiler is activated to generate the steam.

In a further embodiment of the method, the steam boiler assembly comprises the steam turbine. This steam turbine provides that the steam boiler assembly is not only configured to accumulate electricity by using electricity to generate steam, thereby obtaining the pool reward by compensating peaks in the electricity power distribution network. The steam boiler assembly in this embodiment also provides that the steam boiler assembly is configured to generate electricity from the generated steam, by means of the steam turbine.

This embodiment of the method comprises the step of activating the steam turbine when the pool reward for dissipating electric energy is larger than the price per unit of energy for the fuel.

In this situation, the pool reward for dissipating electricity towards the electricity power distribution network is larger than the price per unit of energy for the fuel. It is therefore advantageous to activate the water-tube boiler to generate steam at its maximum production capacity. When the required amount of steam from the steam boiler assembly is lower than the maximum production capacity, a remaining portion of generated steam may be led through the steam turbine in order to generate electricity. This generated electricity may, due to the high pool reward for the dissipating of the electricity, fed back to the electricity power distribution network at a price that is, per unit of energy, higher than the price per unit of energy for the fuel.

In an alternative embodiment of the method, both the water-tube boiler and the electrode boiler may be activated during normal use of the steam boiler assembly when there is no pool reward. In this situation, the steam is simultaneously generated by both the water- tube boiler and the electrode boiler, wherein the electrode boiler is, for example, configured to generate steam at 50% of its maximum production capacity.

If it is determined that there is a pool reward for accumulating electricity, the water-tube boiler is deactivated and the electrode boiler is activated to generate a larger amount of steam, accumulating a larger amount of electricity from the electricity power distribution network, which gives the result that the pool reward is paid to the operator of the steam boiler assembly.

If it is, alternatively, determined that there is a significant pool reward for dissipating electricity, the electrode boiler is deactivated and the water-tube boiler is activated to burn more fuel to generate a larger amount of steam in order to compensate for the deactivation of the electrode boiler. The deactivation of the electrode boiler gives the result that there is no longer electricity consumed, which also gives the result that the pool reward is paid to the operator of the steam boiler assembly, because the power distribution network is relieved. Additionally, the water-tube boiler may be activated to generate an even larger amount of steam, of which a portion may be used in a steam turbine to generate electricity that can be fed to the electricity power distribution network as well, resulting in an increased payment of the pool reward.

The present invention finally provides a method to generate steam, for example for an industrial process, comprising the steps of:

controlling, with a method as described above, a steam boiler assembly,

heating water with a water-tube boiler and/or an electric boiler of the steam boiler assembly in order to generate steam, and

repeating the steps of the controlling the steam boiler assembly and the heating of the water.

Further characteristics of the present invention will be explained below, with reference to embodiments thereof, which are displayed in the appended drawings, in which:

Figure 1 schematically depicts an overview on an embodiment of the steam boiler assembly according to the present invention; and

Figure 2 depicts an embodiment of the method according to the present invention.

Throughout the figures, the same reference numerals are used to refer to

corresponding components or to components, which have a corresponding function.

Figure 1 schematically depicts an embodiment of the steam boiler assembly according to the present invention, generally referred to with reference numeral 1. The steam boiler assembly 1 comprises a steam drum 10, which comprises walls that define an interior of the steam drum 10. The steam drum 10 is configured to hold water and/or steam in its interior, depending on the thermodynamic conditions that are present in the interior of the steam drum 10. Depending on the temperature and pressure in the interior, the water may be present as liquid water, gaseous water vapour or a combination thereof.

The steam boiler assembly 1 comprises a feed water inlet 11 , which is fluidly connected to the steam drum 10. Through the feed water inlet 11 , water may be fed into the interior of the steam drum 10 in order maintain a sufficient level of water that is present in the steam boiler assembly 1. The steam boiler assembly 1 comprises a pump 13 in the feed water inlet, which may be configured to pump the feed water into the interior of the steam drum 10.

The steam boiler assembly 1 further comprises a steam outlet 12, which is fluidly connected to the steam drum 10 as well. The steam boiler assembly 1 is configured to discharge the generated steam, which is temporally stored in the steam drum 10, out of the steam boiler assembly 1 , in order to be used further. The steam outlet 12 may comprise a valve, which is configured to regulate the amount of steam that is discharged through the steam outlet 12.

The steam boiler assembly 1 comprises a water-tube boiler 20 and an electrode boiler 30, which are arranged parallel to each other and which are both fluidly connected to the steam drum 10. The water-tube boiler 20 and the electrode boiler 30 are each configured to increase the temperature of water inside above its boiling temperature, in order to obtain steam, which may be discharged through the steam outlet 12.

The water-tube boiler 20 is, in figure 1 , displayed on the right. The water-tube boiler 20 comprises an inlet 21 , which is fluidly connected to the steam drum 10. The inlet 21 of the water-tube boiler 20 is configured to guide a flow of liquid water from the interior of the steam drum 10 into the water-tube boiler 20, in order to be heated. The water-tube boiler 20 further comprises an outlet 22, which is connected to the steam drum 10 as well and which is configured to allow the exit of the heated stem from the water-tube boiler 20 into the steam drum 10.

The water-tube boiler 20 further comprises a fuel heater 23, which is configured to produce heat by burning a fuel, preferably a fossil fuel. The fuel heater 23 according to the present embodiment is configured to burn a gaseous or liquid fuel like natural gas, which has the advantage that the amount of produced heat by the fuel heater 23 may be accurately controlled and that the fuel heater 23 may be activated or deactivated rapidly, in particular when compared to a fuel heater that is configured to burn solid fuels, such as coal, biomass or the like.

The water-tube boiler 20 further comprises water tubes, which are displayed in figure 1 only schematically, e.g. as a single water tube 24. The water tubes 24 are arranged in a path of the flue gasses from the fuel heater 23, such that the water tubes 24 are heated by the hot gasses that emerge from the burning of the fuel. An entrance end of the water tubes 24 is connected to the inlet 21 of the water-tube boiler 20, whereas an exit end of the water tubes 24 is connected to the outlet 22 of the water-tube boiler 20.

This arrangement provides that water may flow from the steam drum 10, through the inlet 21 of the water-tube boiler 20, into the water tubes 24, where the water is heated under the influence of the hot gasses that emerge from the burning of the fuel and where the water is converted into steam. The steam may continue to flow, through the outlet 22 of the water- tube boiler 20, and back into the steam drum 10, from where it may be discharged through the steam outlet 12 and be used afterwards.

In the present embodiment, the water tubes 24 extend through an exhaust of the fuel heater 23, in order to increase the amount of surface area at which heat is exchanged from the flue gasses onto the water tubes 24. It is understood that, in alternative embodiments, other arrangements of the water tubes may be provided to further increase the amount of surface area between the flue gasses and the water tubes.

In the present embodiment, the water-tube boiler 20 is displayed as a co-current heat exchanger, which implies that a flow direction of the flue gasses from the fuel heater is aligned parallel to a flow direction of the fluid in the water tubes 24, e.g. both being in the upwards vertical direction. In an alternative embodiment, however, the water-tube boiler may be a counterflow or cross flow heat exchanger.

The steam boiler assembly 1 further comprises an electrode boiler 30, which is arranged parallel to the water-tube boiler 20 and which is displayed on the left in figure 1. The electrode boiler 30 also comprises an inlet 31 , which is fluidly connected to the steam drum 10. The inlet 31 of the electrode boiler 30 is configured to guide a flow of liquid water from the interior of the steam drum 10 into the electrode boiler 30, in order to be heated. The electrode boiler 30 further comprises an outlet 32, which is connected to the steam drum 10 as well and which is configured to allow the exit of the heated stem from the electrode boiler 30 into the steam drum 10.

The electrode boiler 30 further comprises a vessel 33, which is arranged in between the inlet 31 and the outlet 32 of the electrode boiler 30. The vessel 33 comprises at least one outer wall, which defines an interior 34 of the electrode boiler 30. The interior 34 of the electrode boiler 30 is configured to hold water, which is fed into the electrode boiler 30 through the inlet 31 of the electrode boiler 30, and steam, which may be obtained by the heating of the water and which may be discharged through the outlet 32 of the electrode boiler 30.

The electrode boiler 30 comprises two electrodes 35, which are arranged at least partially in the interior 34 of the vessel 33, being submerged in the water that is, at least during use of the steam boiler assembly 1 , present in the interior 34. The electrodes 35 are connected to an electricity source 36, which is configured to apply an alternating voltage between the electrodes 35. The water in the interior 34 of the vessel 33 is slightly conductive, which means that the alternating voltage may induce an alternating electrical current in the water. The induced electrical current causes the water in the interior 34 of the vessel 33 to act as a resistor, generating heat. The generated heat may give rise to an increase in temperature of the water, eventually converting the liquid water into steam, which is discharged through the outlet 32 of the electrode boiler 30, back into the steam drum 10, from where it may be discharged through the steam outlet 12 and be used afterwards.

In the present embodiment, the electrode boiler 30 comprises two electrodes 35 between which the voltage is configured to be applied. In an alternative embodiment, the electrode boiler 30 may comprise for example three electrodes, between which a three-phase voltage may be applied. In the present embodiment of the steam boiler assembly 1 , the inlet 21 of the water- tube boiler 20 is fluidly connected to the inlet 31 of the electrode boiler 30. The

interconnected inlets are configured to allow a fluid flow, such as a flow of water, from the inlet 21 of the water-tube boiler 20 to the inlet 31 of the electrode boiler 30 or vice versa.

During normal operation of the steam boiler assembly 1 , only one of the boilers 20, 30 is activated, generating steam, whereas the other one of the boilers 30, 20 is deactivated, not generating steam. The deactivated one of the boilers 30, 20 is not heated and could cool down, while the activated one of the boilers 20, 30 is producing steam. This cooling of the deactivated boiler 30, 20 is undesired, because it drastically increases the required time to switch from one activated boiler to the other. The interconnected inlets 21 , 31 are configured to allow a fluid flow, e.g. a flow of water, from the activated boiler to the deactivated boiler in order to induce a circulation flow of heated fluid in the deactivated boiler to maintain the main volume of water therein at saturated conditions and to prevent the deactivated boiler from cooling down. Depending on which of the boilers 20, 30 is activated or deactivated, the flow in the inlets 21 , 31 may either be directed from the water-tube boiler 20 to the electrode boiler 30 or from the electrode boiler 30 to the water-tube boiler 20.

In the activated one of the boilers 20, 30, for example the water-tube boiler 20, the water is present at saturated conditions, which means that the water is at its boiling point and which further means that all heat that is transferred into the water is converted into steam, e.g. being heated to saturated conditions.

The interconnected inlets 21 , 31 configured to allow a flow of water from the activated one of the boilers 20, 30, in which the water is at saturated conditions, into the deactivated one of the boilers 30, 20 in order to heat the water in the interior of the deactivated one of the boilers 30, 20 to prevent this deactivated one of the boilers 30, 20 from cooling down.

The steam boiler assembly 1 further comprises at least one valve 41. In the present embodiment, the steam boiler assembly 1 comprises two valves 41 , which are each arranged in a respective one of the inlets 21 , 31. The valves 41 are configured to be controlled to guide the fluid flow through the inlets 21 , 31 and are, depending on a control input, configured to either guide the flow to the electrode boiler 30 or to the water-tube boiler 20.

In the embodiment of the steam boiler assembly 1 that is displayed in figure 1 , the water-tube boiler 20 is the boiler that is normally activated, burning the fuel, and the electrode boiler 30 serves the purpose of a secondary boiler. This implies that the water-tube boiler 20 is normally activated and that the electrode boiler 30 is normally deactivated. It is therefore displayed in figure 1 , that the flow through the inlets 21 , 31 is directed to the deactivated electrode boiler 30, in order to prevent the electrode boiler 30 from cooling down during use.

The steam boiler assembly 1 further comprises return conduit 42, which is arranged between the water-tube boiler 20 and a common water inlet 40, in order to allow a fluid flow in between them. The return conduit 42 is configured to allow a return flow of water from the outlet 32 of the electrode boiler 30 to the inlets 21 , 31. The return conduit 42 is thereby configured to return the fluid flow that was introduced in the electrode boiler 30 through the interconnected inlets 21 , 31 , which effectively implies that a loop is introduced between the electrode boiler 30 and the water-tube boiler 20, which bypasses the steam drum 10 and which is configured to prevent the water in the electrode boiler 30 from cooling down.

In an alternative embodiment, however, the steam boiler assembly may be provided without the return conduit 42, which then allows a fluid flow via the common water inlet 40, the electrode boiler 30 and the steam drum 10, being configured to prevent the cooling of the deactivated electrode boiler 30 as well.

The present embodiment of the steam boiler assembly 1 further comprises a controller 50, which is connected to the water-tube boiler 20, e.g. to the fuel heater 23 of the water-tube boiler 20. The controller 50 is configured to control the fuel heater 23 of the water-tube boiler 20, thereby selectively activating or deactivating the fuel heater 23. The controller 50 may for example configured to control a flow of fuel that is fed to the fuel heater 23 to adjust the amount of heat that is generated by the fuel heater. Typically, the controller also controls the respective valves and pumps not shown explicitly in the figures.

The controller 50 is preferably configured to set the fuel heater 23 such, that the water- tube boiler 20 generates an amount of steam that corresponds to an amount of steam that is required to be generated by the steam boiler assembly 1.

The controller 50 is furthermore connected to the electrode boiler 30, e.g. to the electricity source 36 of the electrode boiler 30. The controller 50 is configured to control the electricity source 36 of the electrode boiler 30, thereby selectively activating or deactivating the electricity source 36. The controller 50 may, for example, be configured to adjust the voltage that is applied between the electrodes 35 in the electrode boiler 30, in order to adjust the resulting electric current that is guided through the water in the interior of the electrode boiler 30 and to adjust the amount of heat that is introduced to the water.

The controller 50 is preferably configured to set the electricity source 36 such, that the electrode boiler 30 generates an amount of steam that corresponds to an amount of steam that is desired to be generated by the steam boiler assembly 1.

Finally, the controller 50 of the present embodiment of the steam boiler assembly 1 is connected to the valves 41 and the controller 50 is configured to control the valves 41. The controller 50 is configured to control the valves 41 to guide water, through the inlets 21 , 31 , either into the electrode boiler 30, or into the water-tube boiler 20. The controller 50 may further be configured to control the valves 41 to guide a certain fluid flow, e.g. a determined volume of fluid per unit of time, through the inlets 21 , 31 , corresponding to a desired flow that is sufficient to prevent the deactivated one of the boilers 30, 20 from cooling. The embodiment of the steam boiler assembly 1 further comprises a turbine assembly 60, which is fluidly connected to the steam outlet 12 and the feed water inlet 11 of the steam boiler assembly 1. The turbine assembly 60 comprises a turbine 61 , which is configured to generate electricity, which is indicated in figure 1 by the arrow that is directed in the upwards direction.

The turbine assembly 60 is adapted to receive steam from the steam outlet 12, which induces a rotation of a turbine wheel of the turbine 61 from which electricity may be obtained. For obtaining the electricity, the thermal energy of the steam is lowered, which may result in a lower pressure and/or temperature of the steam and which may even result in condensation of the steam. The turbine assembly 60 is, after the steam has been used to generate the electricity, configured to discharge the finished steam into the feed water inlet 11 , from which it may be fed back into the steam drum 10.

The turbine assembly 60 comprises a valve 62, which is arranged in a conduit between the steam outlet 12 and the turbine 61 and which is configured to regulate a flow of steam from the steam outlet 12 to the turbine 61. By controlling this flow of steam, it may be controlled what fraction of the generated steam in the steam outlet 12 may be used to generate electricity.

The controller 50 is configured to selectively activate the water-tube boiler 20 and/or the electrode boiler 30 in order to generate steam with the selectively activated one of the water- tube boiler 20 and/or the electrode boiler 30. This controlling of the boilers 20, 30 may, in the present embodiment, be done on the basis of a price per unit of energy for the fuel and a price per unit of energy for the electricity.

In figure 2, an embodiment is displayed of the method for the controlling of a steam boiler assembly 1. This embodiment of the method is adapted to be carried out by a controller 50 of the steam boiler assembly 1 , which is indicated in figure 2 by the dashed outline. The method is referred to with reference numeral 100.

In this embodiment, the method 100 comprises a data collection step 110 of obtaining an actual (instantaneous) price per unit of energy for the fuel (Pf) and a data collection step 120 of obtaining an actual (instantaneous) price per unit of energy for the electricity (Pe). These prices per unit of energy (Pf, Pe) may for example be obtained upon request of the controller 50, which is thereto connected, for example by means of a data connection, to suitable resource, for example being the respective suppliers of the fuel and the electricity. It is remarked that the arrows in figure 2, indicating the data collection steps 110, 120, originate outside the dashed outline that represents the controller 50, which indicates that the prices per unit of energy (Pf, Pe) are obtained outside the controller 50.

After the data collection steps 110,120, the method 100 comprises a determination step 130 of determining which of the prices per unit of energy (Pf, Pe) is the lowest. This determination step 130 is in the present embodiment performed by the controller 50, which compares the collected price per unit of energy for the fuel (Pf) to the collected price per unit of energy for the electricity (Pe), e.g. by subtracting Pf from Pe.

The result of the determination step 130 is the lowest price for both energy sources.

For example, when the outcome of the subtraction is a positive number, the price per unit of energy for the fuel (Pf) is the lowest, when the outcome of the subtraction is a negative number, the price per unit of energy for the electricity (Pe) is the lowest and when the outcome is zero, this means that both prices (Pf, Pe) are the same.

The method 100 also comprises a reward data collection step 140 of obtaining a pool reward (PR) for accumulating and/or dissipating electric energy. The pool reward (PR) is, at least in the present embodiment of the method 100, obtained via a data communication connection from the electricity distribution network operator and is a reward for being able to accumulate and/or dissipate electricity within a certain time span and during a certain period. This accumulating and dissipating relieves the electricity distribution network, which normally suffers from peak and valleys in its loading over time.

The pool reward (PR) is, in the present embodiment of the method 100, dependent on the amount of electricity that can be accumulated or discharged, wherein a larger amount of electricity results in a higher pool reward (PR). The pool reward (PR) is further dependent on a reaction time of the steam boiler assembly 1 , wherein a shorter reaction time, e.g. a shorter time between receiving a signal to accumulate or discharge electricity and the actual accumulating or discharging, results in a higher pool reward (PR) as well.

After the reward data collection step 140 the method comprises a correction step 150 of correcting the price per unit of energy for the electricity (Pe) with the pool (PR) reward to obtain a corrected price per unit of energy for the electricity (Pe’).

The correction step 150 is, in the present embodiment of the method 100, carried out by the controller 50 as well and comprises correcting the price of the data collection step 120, in particular the subtracting of the pool reward (PR) from the price per unit of energy for the electricity (Pe). This price (Pe) is thereby reduced, because the pool reward (PR) is a positive number that reduces the net cost of the electricity.

The corrected price per unit of energy for the electricity (Pe’) is then compared with the price per unit of energy for the fuel (Pf), and it is determined whether the corrected price per unit of energy for the electricity (Pe’) or the price per unit of energy for the fuel (Pf) is the lowest.

The method 100 further comprises a steam demand determination step 160 of determining an amount of steam (Qs) that is required to be generated by the steam boiler assembly 1. This amount of steam (Qs) must be delivered by the steam boiler assembly 1 and may be generated either by the water-tube boiler 20, the electrode boiler 30, or by combined heating of both of these boilers 20, 30.

During the steam demand determination step 160, the controller 50 compares the required amount of steam (Qs) with values that are typically stored in the controller 50 and that are representative for both a maximum steam production capacity of the water-tube boiler 20 and a maximum steam production capacity of the electrode boiler 30.

When the required amount of steam (Qs) is zero, the steam boiler assembly 1 does not need to generate steam and, the controller 50 may be configured to deactivate both the water-tube boiler 20 and the electrode boiler 30. Alternatively, the controller 50 may activate either one of the boilers 20, 30 to generate a small amount of steam that is just sufficient to prevent the water in the interior of the boilers 20, 30 from cooling down, thereby maintaining the water at saturated conditions.

When, alternatively, the required amount of steam (Qs) is larger than the maximum steam production capacity of the water-tube boiler 20 or the maximum steam production capacity of the electrode boiler 30, the controller 50 is configured to activate both the water- tube boiler 20 and the electrode boiler 30. The controller 50 is, more in particular, configured to activate the fuel heater 23 of the water-tube boiler 20 and the electricity source 36 of the electrode boiler 30. In this situation, neither one of the boilers 20, 30 is able to generate the required amount of steam (Qs) by itself, which means that the boilers 20, 30 have to co operate and need to generate the steam together.

Preferably, the water-tube boiler 20 and the electrode boiler 30 are activated such, that the steam is generated for the lowest possible cost price. Hence, if the price per unit of energy for the fuel (Pf) were to be lower than the corrected price per unit of energy for the electricity (Pe’), the water-tube boiler 20 is activated to generate steam at its maximum steam production capacity, whereas the electrode boiler 30 is activated to generate the remainder of the steam. If, on the other hand, the corrected price per unit of energy for the electricity (Pe’) were to be lower than the price per unit of energy for the fuel (Pf), the electrode boiler 30 is activated to generate steam at its maximum steam production capacity, whereas the water- tube boiler 20 is activated to generate the remainder of the steam.

When the required amount of steam (Qs) is lower than the maximum production capacity of either one of the boilers 20, 30, the controller 50 may activate one of the boilers 20, 30 to generate the steam. In the present embodiment, the controller 50 is configured to activate the boilers 20, 30 on the basis of the price per unit of energy for the fuel (Pf) and the corrected price per unit of energy for the electricity (Pe’).

When the price per unit of energy for the fuel (Pf) is lower than the corrected price per unit of energy for the electricity (Pe’), it is less expensive to generate steam with the water- tube boiler 20, as compared to when the same amount of steam were to be generated with the electrode boiler 30. In this situation, the controller 50 is configured to activate the water- tube boiler 20 to generate the steam and to deactivate the electrode boiler 30, for as long this has not yet been done.

When the corrected price per unit of energy for the electricity (Pe’) is lower than the price per unit of energy for the fuel (Pf), it is less expensive to generate steam with the electrode boiler 30, as compared to when the same amount of steam were to be generated with the water-tube boiler 20. In this situation, the controller 50 is configured to activate the electrode boiler 30 to generate the steam and to deactivate the water-tube boiler 20, for as long this has not yet been done.

In figure 2, the activating of the water-tube boiler 20 and the electrode boiler 30 is displayed by means of the arrows that project out of the dashed outline of the controller 50, being schematically directed towards respectively the water-tube boiler 20 and the electrode boiler 30.

After the activating and/or deactivating of the water-tube boiler 20 and/or the electrode boiler 30, the method 100 comprises the step of repeating 170 the above-mentioned steps.

By repeating the steps of the method 100, the respective prices (Pf, Pe, Pe’) are repeatedly determined, after which the controller 50 may repeatedly determine which of the boilers 20,

30 may be able to generate the required amount of steam (Qs) for the lowest possible cost price. The controller 50 may thereby repeatedly activate and deactivate each of the boilers 20, 30, in dependence of the prices (Pf, Pe, Pe’), in order to heat the water and to generate the steam.