Field of invention

The present invention relates to a method for controlling district heating networks or grids and more speci fically concerns a method for mitigating the peak heat demand of a thermal power plant associated with the district heating network following the night shutdown or attenuation of the heating systems of buildings connected to the network .

State of the art

The obj ect of the present invention is to reduce the thermal demand peaks in the power plants , which follow relatively long periods of shutdown or attenuation of the heating systems of the connected buildings and to make the network more ready to react to the peaks themselves .

The basic idea o f this technical solution is to present a method for the management of a district heating network such to act on the by-pass valves present in the heat exchange stations of the users or thermal centers , to circulate the water in the primary pipes and therefore heat up in preparation for the thermal demand of all the users , often a plurality of users , downstream of the exchanger, e . g . connected to the secondary circuit of the exchanger . Therefore , in a time interval of lower thermal demand that precedes a time interval of higher thermal demand, the water circulating in the closed circuit is pre-heated in order to accumulate thermal energy useful for the next time interval in which the thermal request of the plant increases .

In particular, the proposed method involves adj usting the amount of energy necessary for heating the network or a part of it at a time prior to the start-up time . The quantity of delivered energy and the delivery processes by the plant are carried out in di f ferent ways e . g . both through an empirical approach and on the basis of historical data, and again through thermo- fluid dynamics simulations of the district heating network, considering the variation over time of the thermal loads of the connected users .

This obj ective is achieved via opening, in a controlled and remote manner, of the by-pass valves present in the network, in particular in the primary water circuit connected directly to heat power plant .

Brief description of the drawings

Further obj ects and advantages of the present invention will become clear from the detailed description that follows , about an exemplary embodiment of the same ( and of its variants ) and from the attached drawings purely for explanatory and non-limiting purposes , in which :

- figure la shows , according to a hydraulic diagram, a district heating network;

- figure lb shows a further hydraulic diagram of a district heating network;

- Figure 2 shows in a comparison chart the results related to the simulation of the network with and without the implementation of this method; and

- Figures 3 to 5 illustrate further graphs of respective results according to di f ferent types of control .

The same reference numbers and letters in the figures identi fy the same elements or components .

The elements and features illustrated in the various preferred embodiments , including the drawings , can be combined with each other without however departing from the scope of the present application as described below . Detailed description The subj ect of this patent application is a method for controlling a district heating network, aimed at attenuating thermal power peaks of the plant / s to meet the thermal demand .

The proposed method is implemented through remotely controlling of by-pass valves arranged along by-pass ducts located in various points of the network ( including substations ) to define one or more closed circuits for the circulation of water to and from the thermal power plant even when the demand for thermal power of the final users is low or zero . The consecutive steps from switching of f to switching on the utility systems can be summari zed as follows .

Purely by way of simpli fied example , reference is made to a fictitious network comprising a delivery pipe , a single user, a return pipe and a power plant . This network includes a primary circuit fed by hot water leaving a heat generation station 1 through a pipe 2 and returns colder water to the station 1 through a pipe 3 . Pipe 2 defines the inlet to a heat exchanger 5 through a valve 4 . The primary circuit is therefore a closed circuit between power plant 1 and exchanger 5 . In general , the primary circuit comprises large diameter flow and return pipes normally arranged side by side when instal led . According to the present invention, upstream of valve 4 , delivery pipe 2 and return pipe 3 are connected by a by-pass pipe with a relative valve 6 that , when closed, connects pipes 2 , 3 via a primary branch releasing heat through heat exchanger 5 and, when opened i f valve 4 is closed, defines a closed loop between heating plant 1 and pipes 2 , 3 . The by-pass duct , depending on the overall configuration of the network, can be placed wherever it is possible to provide, when valve 4 is closed for sending the flow of hot water to one or more users and valve 6 is open, a closed circuit for recirculating the water through the power plant and, therefore, to provide a pre-heating of the water in recirculation while the demand for thermal power is still low or zero. In this way it is possible to provide hot water, i.e. pre-heated, to heat exchanger 5 more quickly when end user 10 requires thermal power. In the example, bypass duct 6 is located in a technical room of the building together with heat exchanger 5, but it can also be located in correspondence with a thermal center of gravity or at another point in the network, to define a closed circuit between a segment of the primary flow pipe and a segment of the primary return pipe. Exchanger 5 comprises a secondary branch, i.e. which receives heat, connected to inlet pipes 8, i.e. at lower temperature and outlet pipes 9, i.e. at a higher temperature after receiving heat, to heat a user 10, e.g. a residential, office, company building etc.

A) In stationary operating conditions, when the user requires a certain thermal power, valve 4 is open. The control unit introduces a flow of hot water at Tdeiivery into the delivery pipe of primary circuit 2. This flow passes through heat exchanger 5, in which it transfers heat to the secondary circuit of user 8-9, and then flows through the return pipe of primary circuit 3 with a lower temperature, reaching the plant with a temperature equal to T _re turn. Consequently, the plant has to provide a thermal flow such as to increase the fluid temperature by AT = Tdeiivery - T _re turn .

B) At shutdown, when the user no longer requires heat power, valve 4 is closed. The flow rate in primary circuit 2-3 is zero (or very low in case of attenuation) . The mass of water present in the pipes stops and begins to cool. In the case of prolonged shutdowns, the decrease in temperature in the network or grid can be very significant, and the AT increases considerably. In the case of Figure lb, a control of the flow rate of the primary circuit is such as to initially slow or stop the flow of water coming from the plant when the request for thermal power is low or zero.

C) At the time of switching on, the flow rate circulating in the network grows rapidly to increase the temperature of the utilities. According to the present invention, it is intended to avoid that, when switched on, the power plant has to provide, in addition to the large flow rate, also a very high temperature difference due to the reduction of the water temperature in the network due to the night transient, which causes a reduction in the temperature of the return flow. For this reason, it is necessary to evacuate (or partially evacuate) the mass of cold water present in the return pipe mreturn = p * V _re turn before activating the utilities.

In order for the mass of cold water to be treated e.g. heated, it is possible to establish the flow rate intended to circulate be fore the actual start-up of the system ( Gbypas s ) ; in this way, in a network with only one user, the evacuation time will be automatically determined as tevacuation Ill return / Gbypas s . Conversely, it is possible to assume the treatment time and calculate the bypass flow accordingly .

D) At this point , the time at which the user will begin to request thermal power again ( t _s tart-u _P ) is known, for example through historical data or through a numerical simulation of the system i . e . o f the variability over time of the plurality of users , it is possible to identi fy the time instant where to activate the new regulation system ( tby- _P as s = t start-up - tevacuation ) . Therefore , at tb _y-P ass , by-pass valve 6 i s opened, while regulation valve 4 remains closed or partially open, ef fectively execution the operation of the primary circuit completely or partially decoupled from the secondary circuit . The by-pass flow rate does not cross the heat exchanger but flows directly from primary circuit delivery pipe 2 to return pipe 3 .

Finally, when the user requests heat power again, valve 4 is opened and by-pass valve 6 closed . The water flows through heat exchanger 5 , supplying the required thermal power to the secondary circuit .

It should be noted that for the purpose of the operation of the invention, valve 4 must be remotely adj usted ( or the corresponding control parameters must be able to be adj usted) and at least valve 4 is controlled on the basis of the temperature measured in the secondary branch of exchanger 5 and / or any other parameter or data that can be acquired to establish the thermal request of plant 1 . The flow rate of the primary circuit is variable , and therefore the circulation pumps also have a variable flow rate , adaptively based on the requests of the various users .

By means of this regulation method, at the moment of actual start-up after step C of pre-treatment performed during a time interval with low thermal demand to the plant , the return temperature to the plant is higher, depending on the actuations performed, compared to that when not applying the method of the invention and becomes equal to the delivery one neglecting dispersions occurred both during shutdown and during operation . In fact , it should be noted that the primary circuit and, in general , the network signi ficantly impact the heating dynamics of the water arriving at exchanger 5 e . g . due to heat losses along the pipes .

According to the present method, therefore , a preheating of the distribution network is provided by bypassing the user . The duration of the preheating and the relative flow rate must be suf ficient to evacuate a signi ficant portion of the mass of cold water present in the return pipes depending on the speci fic system and its operation . In this way, at the moment of actual switching on, the temperature of the water returning to the plant is higher as on average it corresponds to that of the delivery network, except for dispersions along the pipes of the network and in particular of the primary closed circuit . Consequently, the thermal power of the plant decreases .

When the mass of water to be evacuated is known, it is possible to carry out preheating both taking into account the variability over time of the plurality of users and the propagation dynamics of the thermal ef fects between the plant and exchanger 5 : a ) with large flow rates and short times ; b ) with small flow rates and prolonged times .

For a correct evacuation of the mass of cold water, the limitations of the maximum flow rate in each pipe must be respected, in relation to the pressure limitations .

A further advantage of the method is the thermal comfort of the users . Thanks to the preheating of the network, the target temperature of the building is reached much earlier ; in fact , at the time of switching on, the hot water is already present near the building, and the travel time of the hot water flow is signi ficantly reduced . For illustrative and demonstrative purposes , an example is given . In a small network, the application of the proposed invention was simulated through the implementation of a detailed thermo- fluid dynamic model of the network, integrated with a user model . Preferably, the detailed network model allows in particular to estimate the propagation dynamics in the network of the thermal ef fects generated by the power plant 1 .

The model is able to calculate the power profile required by the plant starting from the data relating to the target temperature of the users and the external conditions . The results of the simulation, performed with the hypothesis of evacuation of the mass of cold water relating to the return network in a time of 1 . 5 h, were compared with the results relating to the simulation of the network without the implementation of the proposed method .

Figure 2 shows the load profiles in the two conditions .

The method proposed in the present patent application allows a reduction of the maximum power point of 7 . 7 % . This result was obtained by activating the circulation of water upstream of the primary branch of heat exchanger 5 to allow the water in return pipe 3 to flow through thermal power plant 1 in the 1 . 5 hour immediately preceding the estimated time of increase in the request . The advantages of the present method are even more signi ficant the greater the volume of the pipe sections which become " cold" during the night .

The method obj ect of the present invention operates locally but , preferably and in order to obtain optimi zation, requires information relating to the network in which it is installed . This information can be provided, in addition to considerations on the functioning of the network derived from field measurement campaigns or numerical simulations , through appropriate settings of the control system or through commands sent from a centrali zed system . In general , the control methods intend to obtain the minimi zation of the peak power suppl ied by at least one thermal power plant , defined as the product of the flow rate in the primary circuit , the speci fic heat of the water, and the di f ference in temperature between the water supply and return in the thermal power plant .

Implementation variants of the described non-limiting example are poss ible , without however departing from the scope of protection of the present invention, including all the equivalent embodiments for a person skilled in the art , to the content of the claims .

For example , an arbitrary selection of preheating times and user by-pass flow rates is possible . The latter are selected as fractions of the maximum flow rate allowed by each substation. The advantage lies in the ease of implementation, which does not require knowledge of the characteristics of the network and its operating parameters. On the other hand, knowledge of the switch-on times of the users is required, which determines the thermal request of the plant and is therefore an acquired parameter on the basis of which to start the pre-heating.

Two exemplary cases were tested. In the first test (la - figure 3) , a preheating time of 1 hour and a by-pass flow rate equal to of the maximum flow rate were set for each building. This made it possible to reduce the maximum power to 9.8 MW (-14.6% compared to the base case) , with an increase in thermal energy limited to + 0.21%. It is important to remember that the increase in thermal energy does not correspond to an increase in primary energy, as the surplus is concentrated in instants of time in which there is greater availability of high-efficiency energy sources, while the load is reduced in the peak hours in which it would be necessary to use less efficient resources. Overall, therefore, a reduction in primary energy is obtained, which can be quantified once the production system has been identified.

In the case of the second test (lb - figure 3) , carried out with a greater preheating than in the previous case (2 hours with of the maximum by-pass flow rate) , the maximum power was reduced to 8.6 MW (-25% compared to in the base case) with an increase in thermal energy of + 0.32%.

The method provides for maintaining a minimum flow rate in circulation at all instants of time. This method is simple to apply and manage as it is not required to know the switchon time of the thermal utilities. In fact, the by-pass valve is opened whenever the regulating valve is closed. Two tests were carried out by imposing a by-pass flow rate for each user equal to 1/10 (test 2a - figure 4) and 1/20 (test 2b - figure 4) respectively of the corresponding maximum flow rate. For test 2a, a reduction in maximum power of -17.4% (approximately 9.5 MW) and an increase in energy of 2.26% was obtained. For test 2b, -13.3% for maximum power (equal to 9.9 MW) and + 1.30% for thermal energy.

A further variant involves setting a preheating time and calculating the by-pass flow rates that must be used to dispose of the amount of cold water in the indicated time frame. The calculation of the flow rates is performed through an algorithm that solves a system of non-linear equations. In the event that all users had the same switch-on time, the calculation would be simpler and the identified flow rates would lead to the complete disposal of the quantity of cold water from the return network. In reality, the switch-on in the utilities occurs in a non-contemporary way; the analytical calculation is approximate and the flow rates obtained can lead to excessive or reduced disposal. The algorithm requires knowledge of the topology and characteristics of the district heating network (lengths and diameters of the ducts, incidence matrix, etc.) . In the simulation performed (figure 5) a preheating time of 3 hours was used. With this method it is possible to obtain a reduction in the maximum power of -15.1% (the peak in this case is equal to 9.72 MW) and an increase in thermal energy of + 0.17%.

According to a further alternative embodiment, with respect to the optimization described in the previous paragraph, a correction is introduced on the preheating times, which are no longer set the same in all the users, but are adapted according to the switching times of each of them, with respect to at an average ignition time. The correction is made after the calculation of the by-pass flow rates, for which the same approximation described in the previous paragraph is used. The preheating time used in the test is preferably 3 hours, which are then corrected with respect to the average ignition time. In this way a peak of 9.65 MW is obtained (-15.4% compared to the base case) , with an increase in thermal energy of only + 0.01%.

It is also possible to select flow rates and preheating times through optimization. The optimization is of the Mixed Integer Nonlinear type and can be conducted, for example, through the use of a genetic algorithm. It is possible to choose between different objective functions, such as peak power or daily thermal energy. If the production system (and possibly storage) is known, the optimizer can be integrated with a production system optimizer, and it is possible to set different types of objective function, such as primary energy, the cost of production or total CO2 emissions. Another option is that of multi-objective optimization.

From the above description the person skilled in the art is able to realize the object of the present invention without introducing further construction details.