The present invention relates to the operation of thermal storage systems utilizing off peak storage.

BACKGROUND OF THE INVENTION

Thermal storage apparatuses storing thermal energy in the temperature change or phase change of a suitable medium for example of water to ice are well known. A thermal storage apparatus of this type is described in PCT/AU90/00002 (WO 90/07688), where a series of modules are stacked together to form the storage apparatus.

A phase change medium (PCM), for example water is circulated through the modules in such a manner as to provide a large path through the apparatus. Refrigerant is also circulated through the stack of modules following the water path. This allows a substantial heat exchange in a relatively small 0 volume. In addition, ice can be built up to a much greater depth than in other prior art apparatuses before its growth would obstruct the flow of water through the apparatus. Heat energy stored in the apparatus is utilized by circulating the PCM fluid 5(water) to a load.

At the same time as heat is removed from the water to form ice, the circulating refrigerant removes sensible heat normally by undergoing a phase change from a liquid to a gaseous form. Compression of the gas back ^to the liquid form for recirculation through the apparatus releases the sensible heat which is discharged to the atmosphere.

⁵ In addition, expensive air blowers are often required to create turbulence in the water to effect proper heat transfer and burn off of ice at the required rate. The thermal storage apparatus described in WO 90/07688 avoids the latter difficulty.

The thermal inertia of water as a phase change medium means that there is a delay before the conversion of a given thermal input into ice. This may result in ice blockage preventing circulation of the water and hence the ability to utilize the thermal storage apparatus.

Most prior art refrigeration systems employing a primary refrigerant, for example R22 operate at a refrigerant condensing pressure in a range from 250psi to over 300psi (1670-2120KPa) and suction pressures in the range 27-37psi (253-184KPa) . These pressures are designed to provide suitable pressure drop for thermostatic expansion valves and the like. The need to operate at these high pressures requires large power capacity in the compressor motor which is a disadvantage in terms of cost and energy required.

Most electric utilities sell electricity at a cheaper rate in periods of minimum demand ("off-peak" rates) to encourage its use at these times and the present invention seeks to utilise this cost advantage.

SUMMARY OF THE INVENTION

According to the invention there is provided a refrigeration system comprising a primary refrigerant circuit including a thermal storage apparatus and a surge drum connected in series with a liquid pump to circulate liquid refrigerant between said thermal storage apparatus and said surge drum, and further including a load means which can be connected into

said circuit with said surge drum or said thermal storage apparatus and refrigerant control means controlling the path of refrigerant through said thermal storage apparatus and said load means.

In a first form the refrigerant control means comprises first valve means connected in series between said liquid pump and said thermal storage apparatus, second valve means in parallel with said thermal storage apparatus and said first valve means, third and fourth valve means in series with each other and in parallel with said load means, the outlet from said thermal storage apparatus being connected between said third and fourth valve means.

In a second form the refrigerant control means comprises first valve means connected in series between said liquid pump and said thermal storage apparatus, second valve means connected in series between said liquid pump and said load means, third and fourth valve means in series with each other and in parallel with said load means, the outlet from said thermal storage apparatus being connected between said third and fourth valve means.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described with respect to the figures in which:

Figure 1 shows a refrigerant circuit according to one aspect of the invention;

Figure 2 shows a further thermal energy circuit according to a second aspect of the invention and

Figure 3 shows a schematic of the method for measuring

pressure within the phase change medium' circuit of the thermal storage apparatus shown in Figure 1.

PREFERRED MODES OF PERFORMING THE INVENTION

As shown in Figure 1, a primary refrigerant circuit according to one embodiment of the invention comprises thermal storage apparatus 10, evaporator coil 12 and surge drum 14. In turn surge drum 14 is connected in a second circuit with compressor 16. Surge drum 14 received a gaseous/liquid primary refrigerant from evaporator 12 or storage apparatus 10, separating into gaseous and liquid phases. The surge drum 14 includes a liquid/gas filter screen 40 to better separate the gaseous and liquid phases of the refrigerant. The liquid phase settles to the bottom of the tank and the gaseous phase is liquefied by being drawn from the surge drum 14 through compressor 16 on to the condenser 18 and thence into the liquid receiver 20. From there it returns via the line 22 and valve 24 to the liquid reservoir of surge drum 14.

Liquid refrigerant is withdrawn from the liquid receiver 20 to maintain the level in the surge drum at a predetermined level by opening valve 24 under the control of the photoelectric sensor 26. Other control arrangements, for example using a float device to operate valve 24 can be used within the knowledge of a person skilled in the art.

Liquid is withdrawn from the surge drum via line 28 by the pump 30 driven by motor 31 and thence through pathways which may include the thermal storage apparatus 10 and evaporator coil 12, the path being determined by the positions of the control valves 32, 34, 36 and 38. The mixture of gaseous/liquid refrigerant returns then to the surge drum 14 via line

46 to begin the cycle again.

The circuitry has various modes of operation. With valves 32 and 36 open (valves 34 and 38 closed) liquid refrigerant is circulated through the thermal storage apparatus 10 to charge the thermal storage apparatus. In practice this operation would be performed using off peak electrical energy or demand side management.

To refrigerate evaporator 12 directly, for example in an airconditioned duct or in a cool room, valves 34 and 38 are opened circulating the liquid from the surge drum via the pump 30 and back to the surge drum 14.

To cool evaporator 12 using the stored capacity of the thermal storage apparatus 10 valve 34 is shut and valve 32 is opened, valve 38 also being opened and valve 36 being shut. This mode of operation can be used for example under peak loading to take advantage of the off peak cost of the energy stored in the thermal storage apparatus 10, thereby reducing the overall expense of operating the load of evaporator 12. In this mode the major proportion of the cooling effect in the evaporator 12 is provided by temperature change of the liquid refrigerant, although further cooling can be achieved through phase change of the liquid refrigerant into its gaseous form which is regenerated by the compressor 16 and condenser 18 once the mixture liquid/gaseous refrigerant passes into the surge drum 14.

In an alternative arrangement valve 34 is connected directly with the evaporator 12 by-passing valve 38 as shown in dotted line 49 in Figure 1. With this arrangement it is possible to operate evaporator 12 and thermal storage apparatus 10 in parallel by

opening valves 32, 34, 36 and closing valve 38. This allows charging of the thermal storage apparatus 10 and operation of evaporator 12, for example in off-peak periods. Disconnecting the thermal storage apparatus 10 in this arrangement is achieved by closing valve 32 and at least opening valve 34 or stopping pump 30. Control of refrigerant flow through evaporator 12 in this latter arrangement can be regulated by opening valves 36, 38, which "short-circuits" evaporator 12.

Screen 40 in surge drum 14 needs to be efficient in order to prevent liquid refrigerant from being carried over to the compressor 16. The filter screen comprises a pair of perforated metal screens between which a dense copper screen is located. The perforated metal screen is made of stainless steel, steel or brass depending on the choice of refrigerant. When ammonia (R717) is used brass or copper would not be recommended. It also improves performance if the liquid refrigerant from receiver 20 is passed along the liquid/gaseous line 42 from the surge drum 14 on its return to the surge drum at 44 as shown in dotted line 45. In an alternative embodiment the liquid refrigerant from receiver 20 can be returned to the surge drum 14 via line 46 thereby pre-cooling the returning liquid/gaseous refrigerant as shown by dotted line 47.

Means are provided to feed any oil entrained in the low temperature liquid back to the surge drum and back to compressor 16. Similarly means are provided to feed any oil entrained in the liquid/gaseous refrigerant circuits back to the liquid pump 30, for example by capillary bleed line 48 shown in dotted line in Figure 1.

The thermal storage apparatus 10 as described more fully in WO 90/07688 involves the conversion of a phase change medium for example water into its solid phase.

A liquid refrigerant is circulated through a series of pathways 50 in the apparatus 10 which may comprise a number of layers. To effect the greatest thermal exchange spiral or serpentine pathways are employed. In addition, the phase change medium (water) circulates along the pathway 50 defined by the liquid refrigerant. Baffles or obstructions are provided in the path of the phase medium to induce turbulence and thus effect efficient heat transfer between the liquid refrigerant and the phase change medium. This technique avoids the need to provide an air pump inducing turbulence by air blowing.

The heat energy stored in apparatus 10 is utilised when phase change medium is circulated externally to the storage apparatus 10 for thermal exchange. However, this can only be done provided that circulation of the phase change medium can be maintained and any blockage must be avoided. In order to do this a pressure sensor is included in the phase change medium circuit 61 as shown in figure 3.

A pump 60 circulates the phase change medium through heat exchange 62 and the pressure difference is measured at some point 64 within this circuit. The pressure sensing determines within a predetermined range when a blockage is imminent and when to cease the circulation of refrigerant through the thermal storage apparatus, that is shutting valve 32 and opening valve 34 allowing the refrigerant to bypass the thermal storage apparatus 10. In this manner circulation of the phase change medium is maintained.

Continued circulation within the circuit comprising elements 60, 62 and 64 effects burn off of the built up ice and improves circulation in the storage apparatus 10. At a pre-selected pressure or after a pre-selected time interval the circulation of the liquid refrigerant through the thermal storage apparatus 10 can be reinstated, that is opening valve 32 and closing valve 34. In this way effective utilization is made of the thermal storage apparatus 10 maintaining the ice build at a maximum utilizable level. The capacity of the thermal storage apparatus can therefore be used at all times in both its primary refrigerant circuit and its secondary or phase change medium circuit (as shown in figure 3).

When water is used in the circuit 61 the lowest temperature that can be attained is approximately 0°C. To operate below 0°C a glycol or methanol/water mixture can be used. This allows operation at -1°C to -10°C. Circuit 61 may include an inlet and outlet (not shown) to drain and allow replacement or substitution of the circulating fluid used.

In addition, the condenser 18 provides sensible heat when compressor 16 converts the gas from the surge drum 14 into its liquid state. Thus condenser 18 can be included in a further circuit 70 circulating a suitable fluid whereby the sensible heat provided from the condenser 18 is pumped to a further storage apparatus such as a domestic hot water service storage tank 72 making more efficient use of the heat energies available. In a domestic or commercial environment the thermal storage apparatus 10 and the storage apparatus 72 can be used to provide airconditioning and hot water respectively in the same installation.

The system when using R22 as primary refrigerant

operates in the head pressure range c 1090-1590KPa (159-230psi) and a suction pressure range of 277-346KPa (40-50psi) resulting in substantial energy savings (20%).

Though the invention has been described above with respect to a preferred embodiment, variations are contemplated within the knowledge of a person skilled in the art. For example, the preferred refrigerant at this time is R22, although this can be replaced by other refrigerants depending on the cost and economics of doing this. It is also contemplated that the refrigerant employed may be ammonia (R717).