Systems with high EER, not only at peak load but through the entire range of cooling load (part load) are available for selection. These use high efficiency scroll compressors and have generous coil surface area. The heating application is the one which poses a greater challenge due to thermodynamic constraints of a vapour compression cycle.
A standard heat pump is able to provide reasonable heating capacity during the winter season at ambient above 0º C. However the operating efficiency of the heat pump decreases with decreasing ambient. Also, most heat pumps are unable to deliver adequate heating capacity when the ambient drops below 0º C. Chart #1, the shows the building heating load (green line) and normal heating capacity of a heat pump (red line) plotted against ambient temperature. It is clearly seen that the capacity is inadequate to meet the load. The need is for a heat pump to deliver capacity as per the blue line which will satisfy the load.
The conventional heating options available for sub zero ambient temperatures are;
- Central Boiler System: A large boiler (coal, oil or natural gas fired) generates hot water that is piped to individual homes and offices. Radiators are located in the comfort areas and transfer the heat into the rooms.
- Community Boiler System: This is very similar in concept to the central boiler – with the boiler being located in the community it is designed to serve.
- Individual Home Heater: These are wall hung unitary systems that are used in each home and are gas fired.
- Electric Resistance Heat: Many systems in China have resistance strip heaters that are located downstream of the indoor coil. Depending on the ambient temperature, the strip heat turns on or off.
(The above options have limitations like low efficiency, extra installation, additional piping network, inadequate temperature control, etc.)
- Inverter over speed: Inverter driven heat pump systems can provide additional heating capacity of 20-30% by over-speeding the compressor RPM. While this can meet the heating load demand, EER is very low. Further, the compressor cannot operate for an extended period of time due to overheating.
As can be seen from the above analysis, each heating option has certain merits and demerits but all of them have an overriding problem of delivering heat at low efficiency. The digital scroll heating technology - that uses the enhanced vapor injection principle - addresses the long overdue need of efficient heating solution. This technology is able to deliver additional heat at low ambient conditions in the most efficient manner and also allows the heat pump to operate at very low ambient (up to -20ºC). This has been made possible by the extension of the scroll compressor safe operating envelope by the vapour injection technology.