On Heat Pump Water Heaters (HPWH) we generally are speaking about mono-bloc air source heat pump. This defined as a single unit with heat pump (with compressor, expansion valve, evaporator and condenser) on top and a large storage tank underneath the heat pump. These mono-bloc systems are now and in the future the preferred solution in many cases for single family houses.
Although the monobloc Heat Pump Water Heaters have reached an important level of maturity on the market, it seems that there is still room for improvement of their energy performance.
How does it work?
For an individual HPWH the hot water for the end user is always stored in a tank with sufficient volume in relation to the demand. Crucial are:
- Evaporator, which in the larger part of the market is air source heat exchanger;
- Compressor and expansion valve, dependent on the refrigerant;
- Storage tank.
- Air source heat exchanger as monobloc located with the heat pump on top or under the storage tank, which as unit can be placed indoors or outside
- Air source heat exchanger located as split unit separated from the storage tank, where the evaporator can be installed outside or in a internal ventilation system
- Heat source coupled to a solar thermal system as source, progressively in development are unglazed pvt-systems.
- Low temperature district heating for the booster heat pump.
Other sources are possible.
Especially for Multi Family Buildings a number of building related solutions are marketed for individual air source systems as well as collective ventilation systems.
Original Equipment Manufacturers in Japan use different types of compressors in their ECO-Cute heat pump water heaters using CO2 as refrigerant. Scroll, Swing and Rotary compressors are used. Especially for cold climates the Enhanced Vapor Injection technology for compressors has been introduced.
For the monobloc heat pump water heater there is a number of condenser configurations ranging from the external plate (side arm) heat exchanger, the wrap around heat exchanger and the internal spiral heat exchanger. As a consequence of the need for reduction of refrigerant the external plate heat exchanger is more and more preferred.
Comparison between the types of heat exchanger, the effects of stratification in the storage tank have been tested by Oak Ridge National Laboratories differing for the type of refrigerant used.
An important advantage of external plate heat exchangers is that the overall refrigerant charge can be reduced by optimizing the heat transfer. A great leap forward has been published by KTH from Sweden.
Worldwide storage tanks for individual HPWH (monobloc or split systems) have a capacity often of more than 200 litres for a single family application. However with a larger capacity heat pump the storage size can be much smaller and especially in countries like Netherlands in domestic applications with less space the storage tanks are often 100 – 150 litres.
The efficiency of the storage tank is dependent on:
- Control of the stratification in the tank during draw offs. Efficiencies of 60 – 70 are normal, but up to 90% can be reached
- Control of stratification during heating up process by the heat pump
- Heat losses, which can be very small in well insulated and temperature controlled tanks
- Temperature control in tank.
There are great difference in tank efficiencies.