enEX’ new Ecocute Air 25 unit integrates a control to automatically select the appropriate operation mode of a CO
2 hot water heat pump – either the maximum absolute Coefficient of Performance (COP) possible at actual or a maximum hot water production at a given temperature. The new controller, measuring the amount of hot water stored, can decide if the task of filling the vessels with hot water with maximum COP is on the right track or if more hot water production is needed. This will significantly increase the heat pump’s output and energy savings at the same time.
Moreover, enEX’ new air/water heat pump model Ecocute Air 25 offers the following options normally not present in units of this type:
- A high level of protection of the compressor, measuring during operation oil level, oil pressure and its temperature
- Refrigerant charge control
- Direct control of water temperature, using an high efficiency variable speed water pump, which allows the control of water temperature reducing discharge pressureto a minimum
enEX tests control logic
The control logic integrated into enEX Ecocute Air 25 model passed extensive tests before installation in a residential building. enEX then designed a CO
2 heat pump to meet the tap hot water requirement of a residential building in Northern Italy. The task of the control logic was to identify the couple of variables – water flow rate and gas cooler pressure – that result in the best COP, under the constraint of fixed water temperature at the gas cooler outlet. The self-adjusting logic developed by enEX was able to define its own best working conditions, managing simultaneously the water flow rate and the gas cooler pressure by acting on the pump and the expansion valve.
Test setup & conditions
The heat pump’s basic design consisted of a single-stage semi-hermetic piston compressor, a coaxial type gas cooler, an electronic expansion valve, a finned tube evaporator, and a low pressure receiver. It was combined with a storage tank designed to maintain internal water stratification. Before commissioning it to the residential building, the heat pump was factory tested to verify its energy performance and fine-tune the newly developed high pressure control logic. The estimated hot water consumption was at 6 m3/day of water at 60°C. The heat pump’s water circuit included three storage water tanks connected in series (bottom to top, going from hottest to coldest) to avoid mixing between cold and hot water. enEX’ system was planned for nightly operation with thermal storage.
The heat pump was tested to verify the ability of the control logic to promote the best operating conditions of the inverse cycle at different temperatures water side. Tests were performed at nearly constant air ambient temperature and water inlet temperature of 22°C and 20°C respectively, while the set-point of the water temperature at gas cooler outlet was varied from 60°C to 70°C in steps of 5°C.
Experimental test results
At all water temperatures the response of the system was satisfactory, leading to an experimental COP of up to 4.89 calculated from the performance of the compressor in correspondence of the actual operating conditions. enEX found that an energy-efficient operation of a tap water heat pump requires two conditions:
- To operate with a once-through heat exchanger, so as to impose to water the total lift of temperature from the value at the city system to the one of use. As the COP of a transcritical cycle strongly depends on the refrigerant temperature before the throttling valve, only this arrangement allows this temperature to be the very minimum.
- To optimise the upper pressure of the cycle. As COP depends also on the evaporating pressure and the refrigerant temperature at the exit of the gas cooler, only an adaptive control logic can be used that is able to evaluate the effect on COP of a change in the upper pressure and variations in other variables, under the constraint of constant value of water at the point of use.
About enEX
enEX is specialized in design, prototyping and manufacturing of environmentally friendly air-conditioning, ventilation, refrigeration, and heat pumps. The Italian manufacturer has expertise in complete systems, units, control and regulation systems to measure and optimize performance.
