Thermal storage has been developing for around 25 years. It addresses the particular problem of the temporal dislocation associated with heat sources that provide heat during certain times but when the need for heat is at other times. An example is that of solar heating, where heat is generated during the day, but often needed in the evening or during other periods of low solar gain. By storing heat for future use, the problem is overcome. This technique is often known as ‘buffering’.
A number of methodologies have been developed to store heat over the years, including rock stores and phase-change materials, but the most effective and pragmatic method has been in the use of water (water can retain around 5x more heat than concrete). After a number of years of varying success, the market can now provide efficient water-based thermal stores. Thermal storage tanks appear to be similar in appearance to the conventional hot-water cylinder, but they are very different in their operation.
The principles of thermal storage
A thermal store provides both space heating (radiators or underfloor) and mains pressure hot water.
A thermal storage water cylinder reverses the normal process whereby the boiler heats the water that is to be sent to the taps, this water being stored until required. By contrast, in a thermal storage system, domestic hot water (DHW) is provided via a heat exchanger. Cold water from the mains enters the coil at the top of the tank and is heated by the surrounding hot water before outputting to the taps. Hot water is therefore effectively provided on demand and at mains pressure.
The water that passes through the central heating system also passes through the cylinder.
This water never changes, it simply flows around the vented or un-vented heating circuits before being re-heated through the thermal store.
A boiler, whether supplied by gas, biomass or CHP, is usually the prime generator of heat to a thermal store. Water heated by the boiler passes into the tank and through a heat-exchanging coil and heats the water in the tank.
Additional renewable heating technologies (eg solar collector or heat pump) can be included by adding a further coil to the bottom of the tank-where relatively low-grade heat can be most efficiently employed in heating the coldest part of the tank.
Thermal storage - pros & cons
|Provides effective buffering
|Reduces boiler cycling
|Allows for integration with low temp heating systems eg underfloor
|Adds mains pressure to hot showers
|Provides potable hot water
|The use of a heat exchanger means that in most cases, thermal stores can be integrated with existing pressurised boiler circuits
|Requires much smaller cold water tank then standard vented systems
|Thermal storage is recognised by NHER software
|Heat can be lost through inefficient heat exchangers
|Storage temperature will usually have to be 10 deg C higher than required DHW temperature
|Cannot be used with existing DHW power showers and pumps
|Expensive and unvented storage, very expensive
|Vented stores require a header tank to be located above the heating systems
Points to consider when specifying a Thermal Store
• The design of the heating system should be matched to the calculated peak heat load.
• When including solar heating, ensure that there is extra capacity within the store to accommodate fluctuations.
• Where a biomass boiler is being used, consider sizing the store to provide for the heat capacity generated in a load / firing
• Consider designing not only for short-term anticipated capacity but possible future extensions to the system.
• Consider stratification of water temperatures within the store, particularly where low-grade heating is provided. Effective separation between the hot water at the top of the tank and the cooler water at the bottom, can increase the time between charges.
• Ensure that there is adequate insulation to the store (100mm + PU foam)
• Ensure that there is adequate pipework insulation
• Performance Specification for Thermal Stores, Hot Water Association
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