Refrigerator Troubleshooting Diagram

The four basic heat pump designs for space heating and cooling employ:

• air as the heat source-sink and air as the heating and cooling medium,
• air as the heat source-sink and water as the heating and cooling medium,
• water as the heat source-sink and air as the heating and cooling medium, or
• water as heat source-sink and water as the heating and cooling medium.

Each of these basic designs can supply the required heating and cooling effect by changing the direction of the refrigerant flow, or by maintaining a fixed refrigerant circuit and changing the direction of the heat source-sink media. A third alternative is to incorporate an intermediate transfer fluid in the design. In this case the direction of the fluid is changed to obtain heating or cooling and both the refrigerant and heat source-sink circuits are fixed. The fixed refrigerant circuit designs, generally referred to as the indirect type of application, are becoming increasingly popular, particularly in the larger capacities.

In these systems, coil A in Figure 4.2 is buried underground and heat is extracted from the ground. These heat pump systems have limited use. Practical applications are limited to space heating where the total heating or cooling effect is small, and the ground coil size is equally small. This system requires the burial of several meters of pipe per ton of refrigeration, thus requiring a large amount of land.

Figure 4.2 Types of heat pumps (here, the first part refers to the heat source for the outdoor coil during the heating process and the second part indicates the medium treated by the refrigerant in the indoor coil).

In Figure 4.2, these systems work in reverse of the water-to-air heat pumps: they extract heat from ambient or exhaust air to heat or preheat water used for space or process heating. The system is simply reversed. Heat is extracted from the air inside the home and transferred to water and put back in the ground. All the householder is select the temperature that makes their as cool as they wish.

Figure 4.2 Types of heat pumps (here, the first part refers to the heat source for the outdoor coil during the heating process and the second part indicates the medium treated by the refrigerant in the indoor coil).

These systems use air on both sides (on coils) and provide heating or cooling. In the cooling mode, heat is removed from the air in the space and discharged to the outside air. In the heating mode, heat is removed from the outside air and discharged to air in the space. In these units, it is necessary to provide defrost controls and periods to maintain maximum efficiency. These are the most popular systems for residential and commercial applications because of easy economical installation and lower maintenance cost.

Depending on climate, air-source heat pumps (including their supplementary resistance heat) are about 1.5 to 3 times more efficient than resistance heating alone. Operating efficiency has improved since the 1970s, making their operating cost generally competitive with combustion-based systems, depending on local fuel prices. With their outdoor unit subject to weathering, some maintenance should be expected.

The most popular heat pump is the air-source type (air-to-air) which operates in two basic modes:

• As an air-conditioner, a heat pump’s indoor coil (heat exchanger) extracts heat from the interior of a structure and pumps it to the coil in the unit outside where it is discharged to the air outside (hence the term air-to-air heat pump), and
• As a heating device the heat pump’s outdoor coil (heat exchanger) extracts heat from the air outside and pumps it indoors where it is discharged to the air inside.

Some heat pumps have been designed to operate utilizing a water source instead of an air source simply by designing the outdoor heat exchanger to operate between the heat pump working fluid and water instead of between the working fluid and air. These so-called water-to-air heat pumps have advantages over the air-to-air type if a relatively warm source of water is available which does not require an excessively large amount of pumping power. In particular, industrial waste heat might be used.

In this case, the difference from the water-to-water heat pump is in the method of treating the air. This system provides heating and cooling of air with water as the heat sink or source. The same sources of water can be used in these systems. These systems are less efficient than the water-to-water systems because of the much lower heat transfer coefficient of air. These systems are commonly used in large buildings and sometimes in industrial applications to provide hot or cold water.

The water-to-air heat pumps removes heat from water and converts it to hot air in exactly the same way that a cold water drinking fountain removes the heat from the water and discharges the heat from the side or back of the drinking fountain.

In these heat pump systems, the heat source and the heat sink are water. The heat pump system takes heat from a water source (by coil A) while simultaneously rejecting it to a water heat sink (by coil B) and either heats or cools a space or a process. In practice, there are many sources of water, e.g. waste water, single or double well, lake, pool, and cooling tower. These heat pumps use less electricity than other heat pumps when they are properly maintained. However, without proper maintenance the operating costs increase dramatically.

Table 4.5 shows typical COPs for a water-to-water heat pump operating in various heat distribution systems. The temperature of the heat source is 5°C, and the heat pump Carnot efficiency is 50%.

Table 4.5 Example of how the COP of a water-to-water heat pump varies with the distribution/return temperature.

A systematic classification of the different types of heat pumps is difficult because the classification can be made from numerous points of view, e.g. purpose of application, output, type of heat source, type of heat pump process, etc. If the heat is distributed via a mass flow, e.g. warm air or warm water, this mass flow is called the heat carrier.

Customarily, in the USA heat pumps are classified for the heating of buildings according to the type of heat source (first place) and type of heat carrier (second place). A distinction can be made between the terms:

• heat pump, covering only the refrigeration machine aspect, and
• heat pump plant which besides the heat pump itself also contains the heat source.

This differentiation is due to heat from the heat source being transferred to the cold side of the heat pump by an intermediate circuit, the cold carrier.

Another usual classification differentiates between

• primary heat pumps which utilize a natural heat source present in the environment, such as external air, soil, ground water, and surface water,
• secondary heat pumps which reuse waste heat as heat source, i.e. already used heat, such as extract air, waste water, waste heat from rooms to be cooled, and
• tertiary heat pumps which are in series with a primary or secondary heat pump in order to raise the achieved, but still relatively low temperature further, e.g. for hot water preparation.

Furthermore, heat pumps are generally classified by their respective heat sources and sinks. Depending on cooling requirements, various heat source and heat sink arrangements are possible in practical applications. The six basic types of heat pump are as follows:

• water-to-water,
• water-to-air,
• air-to-air,
• air-to-water,
• ground-to-water and
• ground-to-air.

In each of these types the first term represents a heat source for heating or a heat sink for cooling applications. Schematics of the common types of heat pumps are also shown in Figure 4.2.

Figure 4.2 Types of heat pumps (here, the first part refers to the heat source for the outdoor coil during the heating process and the second part indicates the medium treated by the refrigerant in the indoor coil).

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.

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.

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.