Heat sources

The technical and economic performance of a heat pump is closely related to the characteristics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable temperature during the heating season, is abundantly available, is not corrosive or polluted, has favourable thermophysical properties, and its utilisation requires low investment and operational costs. In most cases, however, the availability of the heat source is the key factor determining its use. The table on the right below presents commonly used heat sources. Ambient and exhaust air, soil and ground water are practical heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal) and waste water are used for large heat pump systems.

Table 1: Commonly used heat sources Heat source Temperature Range (°C) Ambient air -10 - 15 Exhaust air 15 - 25 Ground water 4 - 10 Lake water 0 - 10 River water 0 - 10 Sea water 3 - 8 Rock 0 - 5 Ground 0 - 10 Waste water and effluent >10

• Ambient air is free and widely available, and it is the most common heat source for heat pumps. Air-source heat pumps, however, achieve on average 10-30% lower seasonal performance factor (SPF) than water-source heat pumps. This is mainly due to the rapid fall in capacity and performance with decreasing outdoor temperature, the relatively high temperature difference in the evaporator and the energy needed for defrosting the evaporator and to operate the fans.

  
  

  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 of the heat pump system. Coil defrosting is achieved by reversing the heat pump cycle or by other, less energy-efficient means. Energy consumption increases and the overall coefficient of performance (COP) of the heat pump drops with increasing defrost frequency. Using demand defrost control rather than time control can significantly improve overall efficiencies.

  
  

• 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. Continuous operation of the ventilation system is required during the heating season or throughout the year. Some units are also designed to utilise both exhaust air and ambient air. For large buildings exhaust air heat pumps are often used in combination with air-to-air heat recovery units.

  
  

• Ground water is available with stable temperatures (4-10°C) 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. Open systems should be carefully designed to avoid problems such as freezing, corrosion and fouling. Closed systems can either be direct expansion systems, with the working fluid evaporating in underground heat exchanger pipes, or brine loop systems. Due to the extra internal temperature difference, heat pump brine systems generally have a lower performance, but are easier to maintain. A major disadvantage of ground water heat pumps is the cost of installing the heat source. Additionally, local regulations may impose severe constraints regarding interference with the water table and the possibility of soil pollution.

  
  

• Ground-source systems are used for residential and commercial applications, and have similar advantages as (ground) water-source systems, i.e. they have relatively high annual temperatures. Heat is extracted from pipes laid horizontally or vertically in the soil (horizontal/vertical ground coils), and both direct expansion and brine systems can be used. The thermal capacity of the soil varies with the moisture content and the climatic conditions. Due to the extraction of heat from the soil, the soil temperature will fall during the heating season. In cold regions most of the energy is extracted as latent heat when the soil freezes. However, in summer the sun will raise the ground temperature, and complete temperature recovery may be possible.

  
  

• 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. When large thermal capacity is needed the drilled holes are inclined to reach a large rock volume. This type of heat pump is always connected to a brine system with welded plastic pipes extracting heat from the rock. Some rock-coupled systems in commercial buildings use the rock for heat and cold storage. 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 has the major disadvantage of low temperature in winter (close to 0°C). Great care has to be taken in system design to avoid freezing of the evaporator.

  
  

• 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). Both direct expansion systems and brine systems can be used. It is important to use corrosion- resistant heat exchangers and pumps and to minimise organic fouling in sea water pipelines, heat exchangers and evaporators, etc.

  
  

• Waste water and effluent are characterised by a relatively high and constant temperature throughout the year. Examples of possible heat sources in this category are effluent from sewers (treated and untreated sewage water), industrial effluent, cooling water from industrial processes or electricity generation, condenser heat from refrigeration plants. The major constraints for use in residential and commercial buildings are, in general, the distance to the user, and the variable availability of the waste heat flow. However, waste water and effluent serve as an ideal heat source for industrial heat pumps to achieve energy savings in industry (see also Heat pumps in industry ›››).

 

Last updated: 2008-06-30

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