GB2519742A - Heating Equipment - Google Patents

Abstract

The modular heat store 1 comprises a plurality of heat store modules 2. Each module comprises a chamber holding a mass of phase change material 3 (PCM), a heat exchanger 4 to transfer heat between the PCM and a heat transfer fluid; and connectors (5, figure 1) to enable one or more of the modules to be fluidly connected to one another. The PCM is preferably a hydrated salt having a phase transition temperature of between 30ËšC and 90ËšC. Each module can include an annular cavity 8 closed by a lid 11 to define the chamber. A raised lip 12 permits easy opening of the lid when needed. Each module is ideally less than 20cm in height and can be stacked on other modules. Such an arrangement provides easy installation of the modular store since modules filled with PCM are smaller and less heavy than conventional thermal stores. The modular store can be used as part of a domestic hot water or space heating system, whereby a heat source (15, figure 6A) such as a solar thermal system or a heat pump can provide thermal energy to the store for use later.

Description

Heatin Euuipment

Field of the Invention

The invention relates to devices and systems for storing thermal energy, and also heating systems employing such devices. In particular embodiments, the invention relates to such devices and systems for use in a domestic context, especially in the context of storing heat energy for later use in space heating or for heating or pre-healing water for hol water Jo production.

Background and Prior Art

Intermittent sources of energy production arc a feature of many forms of renewable energy.

is IJnlike fossil fuels, that may be extracted and stored for use as the demand dictates, renewable energy often involves harvesling comparatively low energy density sources such as wind, wave and solar power and converting these into electrical energy that cannot be stored. The supply of energy from these sources is often intermittent and the timing and amount of available power often does not match demand. For example. solar power is productive during hours of daylight, whereas much consumer demand occurs in the evening, or during the night.

Heat pumps are also becoming more commonly used, but the supply from such sources also often fails to match the demand profile of users. There is a requirement, therefoit, to store energy produced by such syslems. For eleclricily-producing technologies such as wind, photovoltaic solar and wave power. excess energy production in times of lower demand can he stored in the form of heat energy. For heat-producing technologies such as thermal solar and heat pump sources, the extracted/harvested heat energy can be stored directly in heat stores.

heat stores, or the concept of thermal storage, is not new in itself. Many systems for storing heat are known such as domestic storage heaters that utilize bricks for storing thermal energy.

Other systems are also known that utilize water as a thermal storage medium, Despite the relatively large heat capacity of waler (approximalely 4kJ/kg/°C), one major drawback with such systems is the large size of storage reservoir that needs to be employed in order to store a practically and conmiercially realistic auiount of energy. Physically large storage facilities are unfeasible or unwelcome, especially in a domestic context. Heat storage systems that use phase change materials have also been considered, utilising the latent heat of the phase change to store energy. Such systems have not been widely used due to challenges with iniplenientation, as well as cost.

It is among the objects of the present invention to attempt a solution to these problems.

S

Summary of the Invention

Accordingly, the inventors provide a modular heat store comprising a plurality of heat store modules, each of said modules comprising: a chamber for holding a mass of phase change material; a heat exchanger, arranged lo lransler heat helween said phase change inalerial, in use, within said chamber and a heat transfer fluid; connectors to enable a heat transfer fluid connection to be made between like modules.

One of the main difliculties ol the provision of heat stores, especially in a domestic context, is is the size of the heat store needed to provide sufficient capacity to provide for the heating needs of the household. Linthed space is often an issue in these contexts, and it is expensive and inefficient for a manufacturer to manufacture and stock a wide range of heat stores of differing capacities so Ihal the working volume olin installed heal store can he oplimised (e.g. maximised) to make the best us of available space. The provision of a modular store allows installers to fine-tune the overall capacity of a system by including more or fewer modules in such a system. By taking this approach, fewer sizes of heat store units need to be produced or stocked.

In situations where the heat store is to be installed with the thermal store material already in place, a complete heal store unit ol any sensible capacity would also he loo heavy br a installer to lift, and manipulate into position. Such a situation might arise where it is not desirable to fill the heat store unit with the storage material after installation. By use of a modular system, the individual modules may be so sized that they can he safely lifted by a single installation engineer, and manipulated into position within the property.

In particularly preferred embodiments, said phase change material is a material that undergoes a solid-liquid phase transition at a temperature between 30 and 90 degrees Celsius. The choice ob using a phase change malerial allows Ihe latenl heal of busion of the material lo he the primary underlying mechanism of heat storage. as well as the specific heat capacity of the is material as its temperature is increased either within its solid or liquid forni. In this situation, the use of the modular design of the heat store becomes particularly effective, because it would normally he difficult lo fill a storage container that formed part ol a heat store with the material once it was in situ in a building. Firstly, the materials would need to he transferred from one container to another causing a risk of spillage within the property with associated damage and perhaps hazard o the installer. Secondly, ii would in principle appear easier o nrnke such a transfer when the phase change material was in its liquid state to ensure that the s storage vessel was properly filled. however, this would require the phase change material to he heated on site for filling. Ibis not only poses practical difficulties, hut further exacerbates the potential risk to property and, especially, to the installer during the filling process. As a result, an adequately large single heat store would need to be prc-fillcd to avoid these problems, but would then be too heavy for an installer to lift and manipulate into position.

Ihe use of a modular heat sU)re avoids these problems by allowing pre-lilled modular heat store units to he produced of a size that may he lifted and manipulated by e.g. a single operator. If slightly larger modules were to he preferred, then a module could readily he produced that could be lifted and manipulated by just a pair of installers working together, is The temperature range for the occurrence of the phase change is most preferably between 30 and 90°C because 30°C is the lowest temperature that would provide a uscablc temperature when the heat was extracted from the heat store and any phase change happening above about 90° would mean the slore would have o operate at that emperawre (or above) when Fully charged; temperatures any higher than this could pose a danger to householders.

Such nmodulcs could e.g. be charged with a phase change material such as tallow, or other lipids or lipid mixtures with a suitahle melting temperature as part of the manuFacturing process, and then shipped, sealed and filled, to the installation position. Such phase change materials are also referred to as organic waxes.

In more preferred embodiments, however, said phase change nmterial comprises a hydrated metal salt. The inventors have found that hydrated metal salts provide particular advantages as phase change materials: they have high thermal storage capacity in terms of hoth latent heat storage capacity and also in terms of specific heat capacity. Such hydrated metal salts arc also of relatively high density (compared to water) thereby giving a greater heat storage capacity for a given volume by comparison to water.

A number of hydraled metal salt materials (and organics) were investigated by the inventors on the basis of melting temperature, latent heat capacity by volume and cost. Of the materials is studied, the following were of particular interest, and hence particularly preferred: Table 1 -Performance of Phase Change Materials Compound Melting Melting Relative Relative Relative Style Temperature Cost' Energy Store cost per per unit per unit kWh volume volume2 stored Sodium Suiphale Incongruenl 32.4 1.0 2.1 1.2 (VI) Decahydrate ______________ ________________ __________ ________________ ___________ Zinc Nitrate Congruent 36.4 7.1 1.5 12.1 Ilexahydrate ____________ ______________ _________ ______________ _________ Calcium Nitrate Congruent 42 3.9 1.8 5.6 Tetr ahydr ate _____________ _______________ _________ _______________ __________ Magnesium Incongruenl 48.4 2.9 2.2 3.4 Sulphate Ileptahydrate _____________ _______________ _________ _______________ __________ Nickel Nitrate Congruent 56.7 8.5 2.5 8.8 Ilexahydrate ____________ ______________ _________ ______________ _________ Sodium Acetate Incongruent 58 1.0 2.6 1.0 Irihydrate _____________ _______________ _________ _______________ __________ Iron Suphate Incongruent 64 3.3 2.7 3.2 Heptahydrate ____________ ______________ _________ ______________ _________ Magnesium Congruent 88.9 1.0 2.1 1.3 Nitrate Ilexahydrate ____________ ______________ _________ ______________ _________ lallow Congruent 39 3.0 1.0 7.8 Commercial Salt Congruent 46 16,4 2.4 17.7 I'CM ____________ ______________ _________ ______________ _________ IleavyParatfin Congruent 60 11.5 1.4 21.3 c:oniniercial Congruent 70 21.5 1.4 40.2 Organic PCM __________ ____________ ________ ____________ ________ Relative cost of materials at UK prices. 2013. The cost is related to availability of materials and the difficulty of their extraction and purification. As a result, this is likely to be (in relative lerms) relalively unchanging. Figures are normalised with respecl to Sodium Acetate Trihydrate.

2 Ibis relates to the capacily of' the malerial to slore energy hy virtue of its latent heal of fusion, calculated on a volumetric basis. Figures arc normalised with respect to the figure for io Tallow.

Hydrated metal salts are generally preferred over organic Phase Change Materials such as fallow, because they have a higher thermal conductivity, thereby allowing heat energy to he transferred into and out of the store at a higher rate.

Ihe "melting style" given in fable I refers lo whether the phase change materials (PCM) change their behaviour on melting and re-freezing. For the particular case of hydrated metal salts, it is know that some of these undergo what is called "incongruent melting" where the salt changes to a lower hydrated form during melting. The two compounds (of different hydraled lorms) have diiferent densities, which leads lo gravily-driven stratification in phase change heat stores, and results in the unavailability (or lower availability) of sonic of the s material for re-freezing; this lowers the effective heat capacity of the system. TIns can, in principle. he addressed by the use of thickening agents to reduce stratification or by the introduction of mechanical mixing. although these approaches have the disadvantage of additional cost and complexity. Additional features of the invention, described below, serve to mitigate these problems in a new and advantageous way.

Of the hydrated metal salts listed in fable I above, the following are particularly preferred, on the basis of their economic and physical characteristics: Sodium sulphate (VI) decahydrate is Zinc nitrate hexahydrate Calcium nitrate tetrahydrate Magnesium sulphate heptahydrate Magesiutn niisale hexahydrate These salts are particularly preferred as the provide a phase transition temperature that is particularly useful for heat inputs from ground source heat exchangers.

Mixtures of such salts are also envisioned, especially euteetic mixtures of the salts. In particular, a mixture of Calcium Nitrate Tetraliydrate and Magnesium Nitrate Hexahydrate, and especially a euleclic mixiure, is envisioned o take advantage of the relalively high latent heat of the Magnesium Nitrate and the lower melting temperature of Calcium Nitrate.

In any aspect ci the invention, it is particularly preferred that the vertical extent of each heat store module (in use) is less than 20cm. Ihe inventors have found the provision of a heat store module with such a relatively small (in use) vertical extent reduces the separation effects observed with incongruent melting hydrated metallic salts.

Most hear slores that are known are provided with at least Iwo heal exchange coils in thermal contact with the heat storage materiaL However, in any aspect of the present invention it is preferred that the heat store modules comprise a single heat exchanger configured to transfer heat both to and froum said phase change umaterial. The inventors have found that such an arrangement not only allows more thermal store material lo he accommodated in a single module, hut that the provision of fewer heat exchange coils also results in decreased costs of the unit. Configuration of the flow paths of the overall heating system allow for such a configuration o he employed. Such configurations are described below.

s Accordingly, the inventors also provide a heating system comprising: a heat source; a heat sink; a heat store as described herein; a heat transfer system to transfer heat via a heat transfer fluid from said heat source to said heat store; a heat transfer system to transfer heat via a heat transfer fluid from the heat store to said heat sink.

/0 Preferably, at least one module in said hear store is configured to have a single heat exchanger, said heat exchanger being used to transfer heat both into and out of said heat store module.

In preferred embodiments of such a system, the heat source comprises a heat pump, and is preferably a ground source heat pump.

Also included within the scope of the invention is a heat store, heat store module and heating system suhsantially as described herein, with reference to and as illustrated by any appropriate combination of the accompanying drawings.

Brief Description of the Figures

The invention will be described with reference to the accompanying drawings, in which: Figures 1 and 2 illustrate schematic cross-sectional view of a heat store of the invention; Figures 3 and 4 illustrate, in perspective view, heat store modules of the invention; Figure 5 illustrates alternative configurations of heat store modules within a heat store; and Figure 6 illustrates, schematically, operational configurations of a heating system of the invention.

Description of Preferred Enibodimcnts

Figure 1 illustrates, in schematic section. a modular heat store according to the present is invention, and generally indicated by 1. The heat store comprises a series of heat store modules 2. The modules are shown in a spaced-apart configuration, for clarity, In operation, ii is preferable that the modules 2 are stacked on top of each other, lo conserve heat. the heat store modules are of generally cylindrical form, the section of Figure 1 being through a diameter of the cylinder, parallel to the cylindrical axis. Each of the modules 2 contains a region containing a phase change material 3. Each module is also provided with a heat exchanger 4. in this embodiment in the form of a coil. The coil is manufactured of a material s suitable to resist the potentially corrosive naturc of the phase change material 3, and might advantageously be formed of stainless steel. Connectors 5 are provided so that modules may he joined to each other in tluid connection such that a heat transfer fluid nmy he passed through successive modules 2. In this illustration, the four modules 2 are connected in series by means of connecting pipework 6. The outer periphery of each module 2 is provided with /0 insulation 7, in order to retain stored heat within the modules. Insulation may also he provided on the top and/or bottom surfaces of modules, in order to insulate the top and/or bottom of a stacked array of modules.

Figure 2 illustrates, in schematic cross-section, a particularly preferred configuration of is modules for a modular heat store 1 of the present invention. The section is taken through a diameter of a cylindrical annular modular. Illustrated arc three heat store modules 2, each of which module provides an annular cavity S in which phase change material 3 can be loaded.

A heat exchanger 4 is provided in the krm of a double helical coil (only the cross-section of the coil elements is shown in Fig 2 for sake of clarity), the heat exchanger being in thermal contact with the phase change nmterial 3 within the annular cavity S. and providing inlet and outlet portions 13 to allow interconnection of heat exchangers between modules.

A lid 11 is provided on the top of each module that may he sealed in place once the phase change material 3 has been put into the annular cavity 8. A raised lip 12 is preferably formed in the lid 11. the outer wall 9 of the module tapers inwardly from top to bottom (in use), as does the inner wall 10. This preferable tapering of the walls allows the modules to be stacked on top of each other, with the base of one module sitting within the lip 11 of a module below, thereby providing additional structural integrity to the stack of niodules The use of an annular cavity in the modules provides number of advantages: The inner walls 10 of the cavity provide structural support to allow the modules to stack on top of each other. The annular shape also eliminates central thermal "dead spots" within the body of the phase change material 3, increasing the effectiveness of the heating and cooling cycles. Also, the annular form allows for a centrally-located clamping mechanism (not illustrated) such as a is rod to he threaded through the array of modules. clamping them together and providing further structural stability.

A layer of insulating material 7 (such as a polyurethane foam) of provided at the inner 10 and outer 9 walls of each module. Further thermal insulation may be provided at the top and/or bottom of the module sLick, and, if required, between each module.

s Figures 3A and 3B illustrate, in perspective view, the upper and lower sides respectively of individual heat store modules 2 of Figure 2, showing more clearly the annular configuration.

Whilst this embodiment is illustrated as a generally circular or cylindrical annulus. it is also envisioned that the modules could be of a more square or rectangular shape. and again preferably of annular configuration. Such a configuration is illustrated, in perspective form, /0 in Figures 4A and 41-3, analogous to the illusftations ci Fig 3 Figures 5A and SB illustrate, in schematic elevation view, different configurations of heat store modules 2 arranged to form a modular heat store I of the present invention. In Figure 5A, all of the modules 2 in the heat slore I are connected in series by connecting /s pipework 6. whereas in Figure SB, the modules 2 are connected in parallel. The connection of modules in parallel affords a faster input and output of heat than series connection. Parallel connection is therefore particularly suitable for installations that require rapid heat output, for exaniple where the syslem is being used to heat or pre-hea water icr domestic ho water requirements. Connection of the units in series affords a slower, hut more long-lasting, transfer of heat in and out of the heat store. As a consequence. series connection is particularly suitable for situations where the system is used to provide long-lasting by low-level heat, such as space heating, and especially under-floor heating systems. The use ci a modular heat store affords the flexibility to configure the individual heat store modules and their interconnections to provide not only a range of overall heat store capacities, but also to provide flexihilily in the heat transfer rales o match any particular installation requiremenL Figures ÔA-6C illustrate schematic arrangements of embodiments of heating systems 14 of the present invention, in different modes of opcration. Each system 14 comprises a modular heat store 1 made up of a plurality of heat store modules 2 (not illustrated individually) as io described herein. The heat store 1 is connected for heat transfer to a heat source 15 and a heat sink 16. In the illustration, the heat sink 16 is exemplified as a space heating requirement. but could equally comprise a heating or pro-heating system for the production of hot water. In this the heat ftansier connections are in the form of pipework 6 wilh a first three-way conirol valve 17 to control flow of heat transfer fluid between the heat source IS and the heat store, and a second three-way control valve 18 to control flow of heat iransfer fluid between the heat store 1 and the heat sink 16. A pump 19 is provided to cause the heat transfer fluid to flow around the system. In Figure 6, "filled" pipework indicates fluid Flow and "open" pipework illustrates a no-flow condition. Similarly the filled elements in the representation of the three-way valves 17, 1 S represent the active flow path through the valve.

In Figure 6A, The system is configured. by a suitable controller, to charge the heat store 1 s with heat derived from a heat sourcc 15 such as a heat pump, a solar thermal system, or even an electrical heater. In Figure 6C, the system is configured to discharge the heat store I thereby providing heat to the heat sink. e.g. in the form of space heating or a system for heating or prc-hcating water. hi Figure OB. the system is configured such that the heat source is providing heat not only to the heat store 1, but also to the heat sink 16, It can he seen that, by using such configurations, only a single heat exchanger. e.g. a single coil, within each module, is required for both charging and discharging the heat store 1.

Figure 7 illustrates a further embodiment of a heat store I of Ihe invention. In this is embodiment, individual heat store modules 2 are provided that are so sixed and shaped as to stack together in a side-by-side arrangement, in use. Individual modules 2 are filled with a phase change material in thermal contact with a heat exchange element (not illusirated) in fluid connection with connectors 5 to enable the modules 2 to he interconnected. In insulation may he provided at the walls of the individual modules 2, or provided as a separate insulating member, such as an insulating jacket surrounding the array of modules. Manifolds 20 may be provided with means to form fluid connections with the connectors 5 of the individual modules, such as push-lit lemale pipe connectors 21. The niani folds may he provided with internal pipeworlc. to allow a required configuration of parallel and/or series heat transfer fluid connections to be made between the individual modules 2. The manifold connections preferably terminate in a single inlet/outlet pipe 22. Nine modules 2 are illustrated in the embodiment of Figure 7. but it is envisaged that as many or as few modules 2 may be used.

depending on the heat storage requirements of any particular system.

I igure 8 illustrates an alternative arrangement of a heat store I of the present invention. In a similar way to the embodiment of Figure 7, in this arrangement individual modules 2 arc shaped so as to fit together in a generally cylindrical fashion, with each module 2 being in the form of a triangular prism or circular segmental prism. Elements in common with those numbered for Figure 7 are numbered correspondingly. In this embodiment six such modules 2 are illustrated, hut it is again envisaged that more or fewer segmental modules 2 may he is employed as required. If fewer modules 2 arc required than would complete the generally cylindrical arrangement shown, insulating blocks similarly shaped and sized to the modules may be inserted into the gaps, to provide structural stability and further insulation to the whole heat store assembly 1.