Water for Heat Source of Heat Pump

Water-source units are common in applied or built-up installations where internal heat sources or heat or cold reclamation is possible. In addition, solar or off-peak thermal storage systems can be used. These sources have a more stable temperature, compared to air. The combination of a high first-cost solar device with a heat pump is not generally an attractive economic proposition on either a first-cost or a life-cycle cost basis.

Ground water is available with stable temperatures between 4°C and 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 be either 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.

Most ground water at depths more than 10 m is available throughout the year at temperatures high enough (e.g. 10°C) to be used as low temperature source for heat pumps. Its temperature remains practically constant over the year and makes it possible to achieve high seasonal heating COPs (3 and more). The pump energy necessary to pump up this water has a considerable effect upon COP (10% reduction per 20 m pumping height). It is necessary to pump the evaporator water back into the ground to avoid depletion of ground water layers.

The ground water has to be of a purity almost up to the level of drinking water to be usable directly in the evaporator. The rather large consumption of water of high purity limits the number of heat pump systems which can make use of this source. Also surface waters constitute a heat source which can be used only for a limited number of applications.

Ground water at considerable depth (aquifers) may offer interesting possibilities for direct heating or for heating with heat pump systems. The drilling and operating costs involved require large-scale applications of this heat source. The quality of these waters often presents serious limitations to their use (corrosive salt content).

Ground water (i.e. water at depths of up to 80 m) is available in most areas with temperatures generally in the 5-18°C range. One of the main difficulties with these sources is that often the water has a high dissolved solids content producing fouling or corrosion problems with heat exchangers. In addition, the flow rate required for a single-family house is high, and ground water systems are difficult to apply widely in densely populated areas. The inclusion of the cost of providing the heat source has a significant impact on the economic attractiveness of these systems. A rule of thumb seems to be that such systems are economic if both the supply and the reinjection sources are available, marginally economic if one is available and not cost-competitive if neither source is available. In addition, if a well has to be sunk, the necessity for drilling teams to act in coordination with heating and ventilation contractors can pose problems. Also many local legislatures impose severe constraints when it comes to interfering with the water table and this can pose difficulties for reinjection wells.

Surface water as rivers and lakes is in principle a very good heat source, but suffers from the major disadvantage that either the source freezes in winter or the temperature can be very close to 0°C (typically 2-4°C). As a result, great care is needed to avoid freezing on the evaporator. Where the water is thermally polluted by industry or by power stations, the situation is somewhat improved.

Sea water appears to be 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 m. 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 minimize organic fouling in sea water pipelines, heat exchangers and evaporators, etc. Where salinity is low, however, the freezing point may be near 0°C, and the situation can be similar to that for rivers and lakes in regard to freezing.

Waste water and effluent are characterized by a relatively high and constant temperature throughout the year. Examples of possible heat sources in this category are effluent from public sewers (treated and untreated sewage water) in a temperature range of 10-20°C throughout the year, industrial effluent, cooling water from industrial processes or electricity generation, and condenser heat from refrigeration plants. Condenser cooling water for electricity generation or industrial effluent could also be used as heat sources. 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.

Apart from surface water systems which may be prone to freezing, water-source systems generally do not suffer from the low-temperature problems of air-source heat pumps because of the higher year-round average temperature. This ensures that the temperature difference between the source and sink is smaller and results in an improvement of the performance of the heat pump. The evaporator must, however, be cleaned regularly. The heat transfer at the evaporator can drop by as much as 75% within approximately five months if it is not kept properly clean. The costs of cleaning become relatively low for larger projects so that the use of this source may become economic.

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