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Heat Pumps Technology Guide SEAI 7. Buffers tanks and thermal stores Key messages • The purpose of a buffer tank is to decouple the operation of the heat pump from the building’s heat load. This optimises both comfort in the building and the heat pump operation. • Thermal stores hold the heat generated by heat pumps for later use. The size of the thermal storage determines what can be achieved with a particular system. Most heat pumps operate with small differences between flow and return temperatures and therefore need large thermal stores. • Heat can be stored under the ground using a ground array. This can be either in consolidated rock using closed loop boreholes or, in an open loop system, in an aquifer. In systems that are supplying a lot of cooling in summer and heating in winter, the design can be optimised to ensure that the system stores as much of the transferred heat as possible. The terms ‘buffer tank’ and ‘thermal store’ are frequently used interchangeably. This is a source of confusion, as their purpose differs between renewable heat technologies. In these guides, the following definitions are used: Buffer tanks The purpose of a buffer tank is to decouple the operation of the heat pump from the heat load in the building, in order to optimise both comfort in the building and heat pump operation. In practice, this means reducing the number of heat pump stop-start cycles under part-load conditions since most heat pumps can only be on (full output) or off (no output). Also, heat pump manufacturers often set minimum run times for their products, during which the generated heat must be removed. The buffer tank meets the heat load in the system and the heat pump turns on and off in a controlled manner to ensure that the buffer is kept sufficiently charged while reducing the number of stop-start cycles. This reduces wear and improves efficiency. Depending on the hydraulic design, a buffer can be connected in parallel with the heat pump flow and return, or just with the return. Thermal stores Thermal stores are used to store the heat generated by heat pumps for later use. While, like a buffer tanks, they reduce the number of stop-start cycles, thermal stores generally operate on a longer time frame, and tend to be significantly larger than a tank simply used as a buffer. Another difference between thermal stores and buffer capacity is the way in which the tank can be connected. A thermal store is connected so that the flow and return from the heat pump and to the heating system are connected to the tank. Where a thermal store is included in a design, it will normally be configured to also perform the role of a buffer tank. Thermal stores enable: • A smaller heat pump to be specified to meet larger peak demands, provided the heat pump has enough time to recharge the store sufficiently between peaks (i.e. the peaks need to be relatively short); • Domestic hot water (DHW) production by heating cold water via a plate heat exchanger that draws heat from the thermal store (typically between 55 oC and 60 oC) on demand. This allows the instantaneous domestic hot water production to be provided at around 45 oC and reduces the risk of Legionella by eliminating the need for domestic hot water storage; and • Heat to be generated at a time when electricity unit prices are lower, such as at night. 29PDF Image | Heat Pumps Technology Guide
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