Integrating Heat Pumps with Thermal Stores
[#Introduction 1 Introduction] [#Heat_Pumps_vs_Other_Heat_Sources 2 Heat Pumps vs Other Heat Sources] [#Temperatures 3 Temperatures] [#Expectations 4 Expectations] [#Schematics 5 Schematics]
[#Split_System 5.1 Split System] [#The_End 5.2 The End]
Where a Heat Pump requires connection to a thermal store (Heat Bank) there are three ways to connect to the store.
Heat Pumps vs Other Heat Sources
- They are limited by the temperatures they can generate, often making them unsuitable for driving higher temperature loads such as radiators, or domestic hot water.
- Their efficiency drops as target temperatures increase, with the greatest efficiencies when delivery cooler water. One must be careful when designing systems that the real-world operating temperatures are not higher than the heat pump is designed for, and that the client is aware of how efficiencies are affected.
- They often have fairly comprehensive controls on-board, including the provision of a domestic hot water cylinder temperature sensor - rather than being switched by more standard (and flexible) external thermostats and switches. This sometimes makes it impossible to apply a controls strategy that makes use of more than one cylinder thermostat.
- Monoblock air source heat pumps require pipework to be filled with anti-freeze protected water to prevent pipe runs to the external unit from freezing. This sometimes makes direct connection to a thermal store unworkable, as the volumes of anti-freeze become excessive, and draining down to service a real problem.
- Heat pumps require a high flow rate of water through them, and hence generate a lower temperature rise, in the order of 5-10°C.
Given that heat pumps work most efficiently at lower temperatures, and thermal stores tend to run hotter than basic domestic hot water stores, there needs to be checks that equipment is compatible.
A thermal store using a coil to generate domestic hot water will require storage temperatures over 70°C to perform well, and will be incompatible with almost all heat pumps without using another form of heat to top up temperatures. Furthermore, if a coil is used to feed the heat pump heat into the store, there will be a further temperature drop, so a heat pump generating 55°C would only really be capable of heating a store to 50°C through a coil.
A better compatibility would be achieved through the use of plate heat exchangers instead of coils, resulting in lower temperature drops. A Heat Bank thermal store, using a plate heat exchanger for DHW, should run from a store as low as 60°C if the PHE is sized correctly. When used in conjunction with a heat pump capable of 55°C, feeding into the store through a PHE, storage temperatures near 53°C would be achievable, resulting in less need to top up temperatures using another source.
It is important to decide exactly what is expected of the heat pump. Some customers expect a heat pump to be capable of driving all their hot water and central heating. Others, who may be more aware of the short-comings of heat pumps, may only expect the heat pump to contribute to heating and understand that hot water needs topping up.
The Following system it possible the most efficient way to integrate a heat pump with a wood burner, making use of a thermal store for primary heat management, and a domestic unvented cylinder to enable high flow rates of DHW from low grade heat.