EFCTC: Heat Pumps Types and Applications
A number of sources are available as heat source
- Ambient air is free and widely available, and it is the most common heat source for heat pumps, but it is less efficient than water-source heat pumps. In addition, in mild and humid climates, frost will accumulate on the evaporator surface in the temperature range 0-6°C, leading to reduced capacity and performance.
- Exhaust (ventilation) air is a common heat source for heat pumps in residential and commercial buildings. The heat pump recovers heat from the ventilation air, and provides water and/or space heating.
- Ground water is available in many regions. Open or closed systems are used to tap into this heat source. In open systems the ground water is pumped up, cooled and then reinjected in a separate well or returned to surface water. Closed systems can either be direct expansion systems, with the working fluid evaporating in underground heat exchanger pipes, or brine loop systems. A major disadvantage of ground water heat pumps is the cost of installing the heat source.
- Ground-source systems are used for residential and commercial applications. Heat is extracted from pipes laid horizontally or vertically in the soil (horizontal/vertical ground coils).
- Rock (geothermal heat) can be used in regions with no or negligible occurrence of ground water. Typical bore hole depth ranges from 100 to 200 metres. Because of the relatively high cost of the drilling operation, rock is seldom economically attractive for domestic use.
- River and lake water is in principle a very good heat source, but care has to be taken in system design to avoid freezing of the evaporator during winter.
- Sea water is an excellent heat source under certain conditions, and is mainly used for medium-sized and large heat pump installations. At a depth of 25-50 metres, the sea temperature is constant (5-8°C), and ice formation is generally no problem (freezing point -1°C to -2°C).
- Waste water and effluent are another source, for instance sewage water, industrial effluent, cooling water from industrial processes or electricity generation, or condenser heat from refrigeration plants. They are mainly used in large projects or for industrial heat pumps.
The last 2 heat sources are preferred sources for district heating and cooling networks, very popular in Northern Europe or in the Netherlands.