Technical Information

Heat pump operation

The closed loop ground source heat pump system consists of three loops that operate during all heat pump cycles and an optional, an additional fourth loop is available that heats domestic hot water, as seen in the following images and defined as:

1. Heating Loop. A loop used to distribute hot or cold energy to the building. For ducted systems a squirrel cage blower is used to move air through a duct distribution system. For under floor heating, this loop will be connected to a collector or series of collectors to circulate the hot water through the property.
2. Refrigerant Loop. A sealed and pressurised loop that transports thermal energy from point to point in the circuit. Refrigerant flow is forced through the circuit by a compressor in the vapour portion of the loop.
3. Ground Loop. A sealed an pressurised loop of water or antifreeze solution circulated below the earth’s surface. It absorbs heat from the surrounding earth in the winter and rejects the heat in the summer. The fluid is circulated via a low power consumption pump.
4. Domestic Hot Water Loop. A sealed and pressurised loop that circulates water from the domestic hot water cylinder to the heat pumps de-super heater circuit. The water is circulated in this loop by a low power circulation pump. Most heat pumps have a full condensing loop which will provide 100% of the DHW requirements.

During heat pump operation, thermal energy is transferred from one loop to another, providing heating, cooling, dehumidification and domestic hot water heating.

Heating Cycle

During the heating cycle the heat inputs and outputs are as follows:

Inputs:

1. Thermal energy from the ground.
2. Heat pump compressor energy.
3. Circulation pump and/or blower energy.

Outputs:

1. Space heating
2. Domestic hot water (DHW)

The major heat inputs are the earth’s thermal energy and compressor energy. For a 10Kw heat pump system operating with a COP of 3, 66.6% of the energy is delivered from the ground and 33.3% from electrical energy inputs. Normally the amount of heat input into the system from pumps will be small compared with the earth and compressor energy. These circulation pumps and blowers will be taken into consideration in the rating of the heat pump capacity and rating.

The refrigerant vapour in the circuit is first compressed, which raises its temperature and pressure. This increased pressure forces the vapour through the refrigeration system.

The hot vapour continues to a refrigerant to distribution heat exchanger which in turn raises the temperature in the building. As heat is removed from the refrigerant in the distribution heat exchanger, the vapour condenses to a liquid. Thus, during the heating cycle, the distribution heat exchanger serves as a condenser.

The warm liquid returning from the condenser passes through a metering device. This reduces the pressure and causes a corresponding reduction in temperature. The low temperature, low pressure liquid then flows to the evaporator where thermal energy from the ground loop vaporises the refrigerant and the cycle begins again.

During the heating cycle a circulation pump moves water/antifreeze solution through the buried ground heat exchanger. As this water circulates through the ground loop, the water is warmed by the earths higher temperature. The heat is transferred from the water to the refrigerant heat exchanger in the water source heat pump. During this part of the cycle the water to refrigerant heat exchanger serves as an evaporator, changing the liquid refrigerant to vapour.

For units with desuperheaters, the hot gas from the discharge of the compressors passes through a second water to refrigerant heat exchanger that heats domestic hot water. At this point, only a small portion of the total heat energy available (the thermal energy of the vapour is in the superheated condition )is removed.

Cooling Cycle

During the cooling cycles, the heat inputs and outputs are as follows:

Inputs:

1. Building heat gain.
2. Heat pump compressor energy.
3. Circulation pump and/or blower energy.

Outputs:

1. Waste heat to the ground.
2. Domestic hot water (DHW).

The heat output during cooling generation will be much greater than that required for domestic hot water heating. A nominal 10Kw heat pump in cooling, the amount of heat rejected back into the ground is approximately 15KW, of this only about 20% would be required for DHW. The remaining heat must be rejected back into the ground heat exchanger to the earth. During cooling operations, the hot gas leaving the compressor preheats the domestic hot water, it is then circulates via the heat exchanger where it is rejected into the earth. Space cooling is achieved by warm air passing through the cold evaporating coil. The high pressure hot liquid in the condenser was forced through an expansion valve which results in low pressure and low temperature. Evaporation of this vapour is the cooling mechanism. The components and their arrangement can be seen on the image below.

Open loop

In the open loop heat exchanger, water is pumped from a well or body of water into a heat exchanger within the heat pump. Heat is either extracted or added by the refrigerant unit and the water is returned to a separate well. In some cases the second, separate well can be a river or small lake.

Until recently these types of systems were the most popular. Where a suitable source of groundwater is available this can still be cost effective as water can be delivered and returned using relatively inexpensive wells.

 However, the main disadvantages of this solution are that water availability is limited, fouling and corrosion may be a problem depending on water quality and most importantly environmental regulations covering the use of groundwater are becoming increasingly restrictive, especially in areas susceptible to drought, making the use of ground water wells inappropriate.

Closed loop

In the closed loop system the ground heat exchanger is made of high density polyethylene pipe, 20mm – 40mm in diameter, which contains a mixture of water and anti-freeze.

After leaving the internal heat exchanger the water flows through a secondary ground loop outside the building to exchange heat before returning. Systems in wet ground or in water are generally more efficient than dryer ground loops since water has a better thermal conductivity than solids in sand or soil.

Closed loop systems can be installed horizontally or vertically, depending on the available space and ground conditions.

The size of the loop depends on the soil type, the average ground temperature and the heat loss/gain characteristics of the building being conditioned.

Vertical closed loop

A vertical closed loop system is composed of pipes that run vertically in the ground.

A hole, typically 100mm – 150mm in diameter, is bored in the ground, typically, 15-120m deep. Pipe pairs in the hole are pre-joined with a U shaped cross connector at the bottom of the hole.

The borehole is commonly filled with bentonite grout to maximise heat transfer. In loose soils bentonite has the added advantage of preventing collapse of the borehole and any water ingress.

Vertical collectors are more expensive than horizontal ones but have a higher thermal efficiency, they are less likely to suffer damage after installation, as there are no backfill requirements.

Bore holes are spaced 5-6 m apart and the depth depends upon the ground and building characteristics. Specialist drilling equipment is normally required, depending on the nature of subsoil conditions.

Potential problems with vertical loops are encountering high water tables and the potential of collapse of the borehole due to unstable soil conditions. These can be overcome with a borehole lining, but this can add significantly to the cost. Obstructions can also significantly increase drilling costs so in most cases some sort of soil investigation, such as drilling trial holes is recommended. A full site survey may be required to avoid damaging existing utility runs.

Some drilling contractors offer to drill inclined boreholes, often under the existing building structure, especially when external space is at a premium. Such solutions, especially with retrofits, should only be considered as a very last resort.

Vertical loops tend to be more expensive than horizontal loops as the use of specialist drilling equipment is a significant proportion of the capital costs involved.

Horizontal loop

A horizontal closed loop consists of pipes that run horizontally in the ground. A long horizontal trench, typically 1.2 – 1.5m, is dug and U shaped or slinky coils are placed inside the trench. Horizontal loops are very common and economical if there is sufficient land available.

A slinky closed loop system, also known as a coiled system, is a type of horizontal closed loop where the pipes overlay each other. They can be easily made up on site and are used where there is not adequate room for a true horizontal loop.

Slinky loops can be anywhere from one third to two thirds shorter that traditional horizontal loop trenches. However, this must be balanced against pipe lengths that may need to be doubled to achieve the same thermal performance.

The actual performance of the heat pump system is a function of the water temperature produced by the ground loop (which depends upon the ground temperature, pumping speed and design of the ground loop) and the output temperature.

 It is therefore essential to design them together.

The most important first steps in the design of a GSHP are:

·         the accurate calculation of the building’s heat loss.

·         the estimate of the building’s energy consumption profile.

·         domestic hot water requirements.

·         any other system requirements e.g., swimming pool heating.

Collecting accurate data at this stage will allow accurate sizing of the heat pump system. This is particularly important because the capital cost of a GSHP system is generally higher than for alternative conventional systems.

Oversizing will significantly increase the installed cost for little operating saving and will mean that the period of operation under part load is increased. Conversely if the system is undersized design conditions may not be met and the use of expensive top up heating will reduce the overall system efficiency.

The design of GSHPs is a specialist activity and should only be contracted to qualified GSHP designers. Under no circumstances should clients permit contractors to design GSHP systems unless they have access to an accredited GSHP designer.