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10.4 Heat pumps

Dedicated heat pumps

A refrigerant system can be used to heat residential or commercial areas by utilizing the condensation energy. Heat required by the evaporation comes from the surroundings through a loop of brine in the ground (see Figure 10.8) or from a lake, the ambient air or the bedrock. Dedicated heat pumps are common in cold countries where the low requirement for air conditioning during the summer makes reversible heat pumps less attractive. By optimizing the heat pump for heating only, the operating cost will be lower than for reversible heat pumps. Dedicated heat pumps can also be used to produce hot tap water by increasing the condensing temperature when hot water is required or by combining the system with a desuperheater.

Dedicated heat pumps operate with relatively low temperature differences in both evaporator and condenser to maximize the coefficient of performance (COP). The use of a SWEP distribution device is highly recommended to maximize efficiency. The system performance can also be improved by using a separate condensate sub-cooler or liquid suction heat exchanger.

Residential heating by heat pumps is supported by the governments of some countries as a step towards higher energy efficiency and a reduction in "dirty" heating, e.g. with local oil/coal burners.

Reversible heat pumps

Reversible heat pumps can be utilized to provide both air conditioning and heating, depending on the requirements. The operation of the compressor and secondary fluid is constant, but the refrigerant flow is reversed by a four-way valve. Figure 10.9 shows reversible heat pump systems with a BPHE working as a condenser and an evaporator, respectively.

Because the direction of flow of the secondary fluid is constant, the heat exchange for the evaporator/condenser will be parallel for one operation. Parallel, or co-current, flow gives a lower mean temperature difference and thus lower performance. The decision to have the evaporation or condensation operate in parallel therefore depends on the climate and thus the system requirements. Normally, the evaporator is operated in counter-current and the condenser in parallel.

SWEP evaporator models with distribution systems have a higher efficiency than those without, and they operate without problems when reversed, i.e. in condenser duty. The pressure drop induced by the distribution device is negligible when only liquid passes through, as is the case when operating as a condenser.

Exhaust air heat pump

In houses with ventilation systems in cold climates, the warm exhaust air can be used to operate a heat pump. The use of highly efficient BPHEs as condensers makes it viable to utilize the warm air to evaporate the refrigerant in an air coil before releasing it. This minimizes the waste energy (see Figure 10.10).

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