More efficient air-to-water heat pumps thanks to EVI compressors
Our experience shows that more and more people ask us about possibilities of modernization of their traditional radiators heated by expensive, 'gas-devouring' boilers. This requirement has become solvable. For these potential buyers Hotjet CZ. s.r.o. has developed a new generation of heat pumps equipped with EVI (Enhanced Vapour Injection) compressors. In this article you can read a short overview of heat pumps equipped with non-EVI and EVI compressors and about main differences between their operation. You will be informed also about adventages of the EVI compressors. We hope we can help you in your decision making process if you think whether to change your gas boiler to a heat pump system or not. Of course, the EVI-technology can be used in heating systems of new houses, too.
It is well-known that a heat pump system can be economically operated only in case if it has been designed precisely to work for a low-temperature heat distribution system. In case of traditional, non-EVI compressors this practically means flow temperatures between 35 and 45 °C which is sufficient for floor-, ceiling or wall-heating systems and fan-coils. You can ask why this low temperature is insufficient in case of radiators? The answer is that the heat radiating surface of the radiators is much smaller than the surface of the low-temperature heat distributors. To be able to radiate the same heat amount through smaller surfaces, the temperature of the heating water must be higher. But in parallel with increasing temperature differentials between the heat source and the heating water, the COP of the heat pump will fall down. For example, in case of an air-to-water heat pump you want to produce higher and higher water temperatures when the outside temperature falls and the heat loss of the house increases. Shorly saying, the temperature differential between the air (heat source) and the heating water becomes bigger and bigger which means you will be able to extract a bigger amount of heat energy only in case your heat pump consumes more electrical energy. In consequence its efficiency falls. This is physics and nothing else. In case of heat pumps equipped with non-EVI compressors this problem can be managed only if we employ low-temperature heat distributors. But for radiators higher water temperatures are needed. Generally in old buildings gas boilers and radiators are installed and their change is often impossible. In such cases heat pumps with EVI-compressors can serve as the solution if you want to modernize your heating system.
If we compare heating powers of the EVI-compressors with traditional ones, we find that EVI-compressors have higher power outputs at the same outside temperatures as it is shown in figure 1. You can see that in low temperature range, lower than 0 °C, decrease of power output of EVI-compressors is less than that is of the traditional ones. In other words, the power output of the EVI-compressors increases when it is really needed because of the falling outside temperature. This feature makes possible to serve as a heat generator for traditional radiators so that the COP remains favorable thanks to special refrigeration circle with vapor injection. Installation of EVI-compressors is especially important in case of air-to-water heat pumps, power outputs of which are strongly dependent on the outside air temperature. This dependency will not be fully eliminated but it will be weakened.
Figure 1. Changes of power output plotted against outside air temperature in case of traditional and EVI-compressors
Translations: Fűtő teljesítmény - (heating) power output, előremenő hőmérséklet - flow temperature of heating water; Hagyományos hőszivattyú - traditional heat pump; Épület hőigénye - heat loss of the building; Külső hőmérséklet - outside temperature
Adventages listed above are explained by the construction of the refrigeration circle that differs from the circle of the traditional compressors (see figure 2). The main differences are represented by an additional heat exchager, called economizer, and an additional pipeline.
Figure 2. Principle of EVI-cicle
Translations: M+g tömegáramú hűtőközeg - Refrigerating gas with flow rate of M+g; Kondenzátor - Condenser; Mágnesszelep - electromagnetic valve; Gőzbefecskendezés - Vapor injection; g tömegáram - flow rate g; Fojtószelep - expansion valve; Elpárologtató - Evaporator; M tömegáram - Flow rate M
The refrigerant is divided in two flows after the condenser - a bigger flow M and a smaller flow g. Flow g goes through an expansion valve and its pressure and temperature fall down. Then this flow g gains additonal heat from the flow M in the economizer, this way overcooling it, and at the same time flow g evaorates. So, flow g can be injected into the compressor and also becomes usable to cool the compressor.
Thanks to overcooling of the refrigerant and cooling of the compressor, EVI-compressors can increase their power output and they become suitable in heating systems with traditional radiators. At the same time, they can be connected to low-temperature heat distributors, too. Their COPs are generally higher than those of their traditional competitors, making this way the operational cost more favorable. If you have any questions, please feel free to contact us.
