Enhanced Vapor Injection for Air to Water Heat Pumps

What is Enhanced Vapor Technology (EVI)?

Enhanced Vapor Injection (EVI) is a method of increasing the saturation temperature/pressure gap between evaporator and condenser, by using an economiser and a secondary expansion valve, in order in increase the leaving hot fluid temperature in heating mode (Air or Water).

Lowering the saturation temperature in the evaporator (outdoor unit in heating mode) allows it to absob more heat from ambient air at lower climatic conditions. Also having a refrigerant superheat prior of refrigerant injection into compressor, increases the compressor discharge pressure and temperature (which increases the temperature of fluid leaving condenser)

Hydro Solar HSS series Air to Water (ATW) Heat Pumps, use DC inverter Panasonic compressors. These compressors are variable speed compressors powered by direct current inverters. Speed is modulated via an external variable frequency drive - to control the speed of the compressor. The refrigerant flow rate is modulated by changing compressor's speed. The turndown ratio depends on the system configuration and manufacturer. It modulates from 15 or 25% up to 100% at full capacity. This means that heat pump operating with a DC inverter compressor can easily matche its capacity to the demand by simply modulating its compressor speed. Unlike conventional one or two stages compressors, Heat Pumps equipped with a DC inverter compressor do not cycle ON and OFF, they run continously at lower speeds.

As shown in the above schematic, the liquid out of the condenser is separated into two parts. A smaller part of the liquid (i), is expanded through an additional expansion valve, and then directed (or flows) into a counter-flow plate heat exchanger, HX. The main part of the liquid out of the condenser (m), is then cooled down through the economizer while evaporating and superheating the injection mass flow. This additional plate heat exchanger, more generally called economizer, acts therefore as a sub cooler for the main mass flow (m) and as an evaporator for the injection mass flow. Superheated vapor is then injected into the intermediate vapor injection port in the scroll compressor.

The additional subcooling increases the evaporator capacity by reducing the temperature of the liquid from TLI to TLO, thus reducing its enthalpy. The additional condenser mass flow (i), increases the heating capacity by the same amount.

Efficiency with vapor injection scroll compressor cycle is higher than that of a conventional single stage scroll delivering the same capacity because the added capacity is achieved with proportionally less power. The injection mass flow created in the subcooling process is compressed only from the higher inter-stage pressure rather than from the lower suction pressure.

The additional Sub-cooling effect of EVI configuration allows heat pump to draw heat from the outdoor at lower outdoor temperatures. That could explain why DC inverter (Non EVI) Heat Pumps operate between -20⁰C and 45⁰C (Outdoor BD Temperatures) while DC Inverter EVI Heat Pumps operate between -25⁰C and 45⁰C (Outdoor BD Temperatures).

Comparison of Air to Water Heat Pumps with and without EVI (Enhanced Vapor Injection) Technology

In this blog, we will be comparing the HSS030V2 Monoblock Heat Pump (Without EVI Technology) with the HSS030V2LM (With EVI Technology. The compared heat pumps have similar nominal cooling and heating capacities. The non EVI HP has an outdoor temperature operating range from -25⁰C to +45⁰C while the Non EVI HP has an outdoor temperature operating range from -20⁰C To +45⁰C.

In general, ATW heat pumps have different outputs, power inputs and COP at various supply water temperatures. Comparisons in this blog will be presented for supply temperatures of 45⁰C and 55⁰C.

Capacity:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have superior heat output than the ones without the EVI ones.

Power Consumption:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have a higher power consumption than the ones without the EVI ones.

Power Consumption:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have a similar COP than the ones without the EVI ones.

Capacity:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have superior heat output than the ones without the EVI ones. The performance surplus for 55⁰C water supply temperature, between EVI and Non-EVI Heat pumps, is larger than for 45⁰C water supply temperature.

Power Consumption:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have a higher power consumption than the ones without the EVI ones. The power draw surplus for 55⁰C water supply temperature, between EVI and Non-EVI Heat pumps, is larger than for 45⁰C water supply temperature.

Power Consumption:

At temperatures below -7⁰C, ATW Heat Pumps with EVI technology have a higher COP than the ones without the EVI ones. 

Conclusion

Air to Water Heat Pumps with EVI technology have a higher performance and higher heating energy output at lower outdoor temperatures at higher water supply temperatures. This makes them a better choice for Nordic Climates such as Canada and the Northern portion of the United States.

Air to water heat pumps without the EVI technology are suitable for milder climates with significant heating demand such as the central portion of the United States (from East to West).