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Archive for the ‘Heat Source’ tag

Solar for Heat Source of Heat Pump

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Solar energy, as either direct or diffuse radiation, is similar to air in its characteristics. A solar-source heat pump or a combined solar/heat pump heating system has all the disadvantages of the air-source heat pump, low performance and extreme variability with the additional disadvantage of high capital cost, particularly as in all cases a heat-store or back-up system is required. In areas with high daily irradiation, this may not be the case.

Each of the above mentioned heat sources for heat pumps presents some drawbacks. Presently considerable research is devoted to the technical problems involved and alternative heat sources. Also solar energy may provide a suitable heat source. Unfortunately, solar systems presently are very costly. Furthermore, the intermittent character of solar energy requires the use of large and costly storage volumes.

Written by sam

January 30th, 2010 at 6:06 pm

Soil and geothermal for Heat Source of Heat Pump

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Soil or sub-soil (ground source) systems are used for residential and commercial applications and have similar advantages to water-source systems, due to the relatively high and constant annual temperatures resulting in high performance. Generally the heat can be extracted from pipes laid horizontally or sunk vertically in the soil. The latter system appears to be suitable for larger heat pump systems. In the former case, adequate spacing between the coils is necessary, and the availability of suitably large areas (about double the area to be heated) may restrict the number of applications. For the vertical systems, variable or unknown geological structures and soil thermal properties can cause considerable difficulties. Due to the removal of heat from the soil, the soil temperature may fall during the heating season. Depending on the depth of the coils, recharging may be necessary during the warm months to raise the ground temperature to its normal levels. This can be achieved by passive (e.g. solar irradiation) or active means. In the later case, this can increase the overall cost of the system. Leakage from the coils may also pose problems. Both the horizontal and vertical systems tend to be expensive to design and install and, moreover, involve different types of experts (one for heating and cooling and the other for laying the pipe work).

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 m. 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.

The ground constitutes a suitable heat source for a heat pump in many countries. At small depth temperatures remain above freezing. Furthermore the seasonal temperature fluctuations are much smaller than those of the air. Heat is extracted from the soil by means of a glycol solution flowing through tubing embedded in the ground. If a horizontal grid of tubing is utilized, several hundred square meters of surface area are needed to heat a single family building. In urban areas such space is rarely available. In addition considerable costs are involved. For these reasons vertical ground heat exchangers are more preferred presently.

Geothermal heat sources for heat pumps are currently utilized in various countries, particularly in the USA, Canada and France. These resources are generally localized and do not usually coincide with areas of high-density population. In addition, the water often has a high salt component which leads to difficulties with the heat exchangers. Due to the high and constant temperatures of these resources, the performance is generally high.

Written by sam

January 30th, 2010 at 6:05 pm

Water for Heat Source of Heat Pump

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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.

Written by sam

January 30th, 2010 at 5:54 pm

Air for Heat Source of Heat Pump

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While ambient air is free and widely available, there are a number of problems associated with its use as a heat source. In the cooler and more humid climates, some residual frost tends to accumulate on the outdoor heat transfer coil as the temperature falls below the 2-5°C range, leading to a reduction in the capacity of the heat pump. Coil defrosting can be achieved by reversing the heat pump cycle or by other less energy efficient means. This results in a small energy penalty because during the defrost cycles cool air is circulated in the building. Provided the defrost cycle is of short duration, this is not significant. In addition, for thermodynamic reasons the capacity and performance of the heat pump fall in any case with decreasing temperature. As the heating load is greatest at this time, a supplementary heating source is required. This device could be an existing oil, gas or electric furnace or electric resistance heating; the latter is usually part of the heat pump system. The alternative to the provision of a supplementary heating device is to ensure that the capacity of the heat pump is adequate to cope with the most extreme weather conditions. This can result in over-sizing of the unit at a high additional capital cost and is not costeffective compared with the cost of supplementary heating devices.

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 utilize 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.

Outside ambient air is the most interesting heat source as far as availability is concerned. Unfortunately when the space heating load is highest the air temperature is lowest. However, temperatures are not stable. The COP of vapor-compression heat pumps decreases with decreasing cold source temperature. In addition at evaporator temperatures below 5°C air humidity is deposited on the evaporator surface in the form of ice. This does not improve the heat transfer and leads to lower working fluid temperatures and therefore lower COP values, depending upon the temperature of the air flowing over the evaporator. If ice formation occurs periodic de-icing of the evaporator surface has to be applied. This invariably leads to decreased values of the overall system COP (5–10%).

Written by sam

January 30th, 2010 at 5:45 pm

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