A study of large air to air heat pumps in a clothing store

This report describes the field trial undertaken at a Manchester clothing store with a gross floor area of 5950 m2, served by eight rooftop air to air heat pumps. The heat pumps have a design heat output of 61 kW each, and the data complements that which has been derived from earlier trials with smaller units. A description is given of the store and its services, with the instrumentation installed. Detailed instrumentation was applied to one representative floor, and the overall pattern for the store derived using this data and the known total loads and operating hours. The energy consumptions are shown for a period of one year, with typical demand curves and the annual running costs. The information obtained is discussed with reference to previous heat pump field trials and to field trials carried out in two other stores in the same chain employing central plant air conditioning and oil heating. It is shown that the total electrical energy was similar at Manchester to the other stores, but that the heating energy was considerably lower, due to the lower specific heat requirement. The predominant requirement was for cooling, which in the winter months is satisfied by free cooling. Coefficients of performance for the heat pumps were found to be similar or slightly lower than others previously measured with a seasonal heating COP of 2. 3 - 2.4. The importance of maintenance was highlighted by one of the eight heat pumps whose performance was observed to fall off under a fault condition, yet without the machine tripping out. Attention is drawn to the distribution energy for the store, which at 68.6 kWh/m2 per annum contributed 35% of the electrical energy, and which it is felt could be reduced if a different approach to ventilation design were adopted. Running costs for the store totalled £6. 70/m2 for the year July 1979 to June 1980.