Heat Pumps

A heat pump utilises energy from natural resources such as the ground, the air and water. It does not diminish any of these natural resources or produce waste by-products and is therefore deemed a renewable product.

This energy from mother nature is transformed by electricity to generate up to 4 times its power output. For every 1kw of electricity provided up to 4kw's are "free". No other heat source can provide such impressive efficiencies. The key characteristic of heat pumps over traditional boilers is that the amount of energy produced is greater than the energy used to drive the process.

The heat pump works in the same principle as a refigerator with the exception that the heat generated is utilised. Refigerators take heat from the food and release into the room through the back of the unit. In the same way a heat pump obtains heat from the air, earth or water and "pumps up" to a temperature level required to heat a house. A heat pump is at its most efficient when the energy derived from mother nature is at its highest and the flow temperature leaving the heat pumps is at its lowest e.g 35 -40ºC. The preferred heat medium is underfloor heating although high efficiency aluminium radiators and fan coil convector's can be used for the bedrooms.

How do heat pumps work?

A heat pump only needs a heat source (the ground, air or water), two heat exchangers (one to absorb and another one to release heat) and a relatively small amount of drive energy to keep the system going. A heat pump extracts thermal energy from the environment. The pump extracts the energy at a certain temperature, increases that temperature and then releases it into a medium which is the water running to your low temperature radiators, under floor heating system or fan coil units. Between those two media the heat is moved by means of a working fluid.

Compressor - the essence of heat pumps

As the working fluid passes through the evaporator and extracts heat from the energy source it turns into a gas. This is where the compressor comes in. When you compress a gas the temperature rises. (for example if you inflate the tyre of your bicycle, you can feel the air inside warming up through the rubber). Inside your house the second heat exchange takes place when the compressed gas enters the condenser, a surface which is colder than the gas itself. Finally, the gas condenses and releases its heat - this heat then warms up your house.

It is important to emphasise that the practical benefits of heat pumps can only be realised if the overall system is properly designed, installed and commissioned. Key factors in maintaining efficiency are:

• Minimising the temperature difference between the source and the sink i.e selecting the warmest possible source and and used the lowest heat distribution temperatures.
• Avoiding the over or under sizing of the heat pump package.
• Ensuring an apropriate control strategy and good control for space heating and hot water.
• Ensuring the correct thermal sizing and hydraulic design of the ground loop system.
• Ensuring that economy electrical tarriffs are available.

ENERGY SOURCES:

1. Horizontal/Flat Collector.

The horizontal collector is the most common option for domestic installations due to the lower installation cost. 25mm HDPE are placed in trenches throughout the garden area. The trenches are 50metres long, 1.2metres deep and 1.0metre wide. The HDPE pipe is laid out in 100metre rolls and connected to a distribution manifold in a concrete ring. The flow and return is then connected to the heat pump.

Different soil types have different thermal capacities with a heavy saturated clay soil providing the best output 40 W/m² and a dry sandy soil providing the worst 10W/m² (VDI4640).

2. Borehole/ Vertical collector.

Borehole collectors are usually used in commercial applications or were the client has no space available for a horizontal system but are generally more expensive. Boreholes range in depths from 100metres to 180metres and should be a minimum distance apart of 6metres to prevent intersection. U tubes made of 40mm SDR11 high-density polythene pipe are placed into the boreholes and filled with an anti-freeze brine mixture. Boreholes are generally back-filled with a betonite grout mixture to enhance thermal conductivity. Under no circumstances should the borehole have an air gap around the pipe as this will affect the heat transfer and borehole performance.

Multiple boreholes are connected to a ready accessable manifold chamber with flow meters and isolating valves. The connecting flow and return pipework to the heat pump are buried in a 1.2metre trench and insulated within 1.0metre of the building.

Again the number and depth of boreholes are relative to the system requirement and the size of the heat pump and are calculated at design stage. Different rock types provide different thermal capacities with granite providing 65-85 W/m² and Basalt 40-65 W/m² ( VDI 4640).

3. Water to water.

Water to water boreholes offer the most efficient systems but also potientially the most problematic. In essence you require a plentiful water supply and only two boreholes are used; water is extracted from one and returned to the other through the heat pump to maintain the water table. Temperatures coming out are usually a constant 10°C.

Boreholes are usually about 30 - 50metres deep with a submersible pump down the source hole pumping the water through the heat pump. The boreholes must be kept a minimum distance of 15metres apart with the water directional flow from the source borehole to the return borehole to prevent cross contamination.

Important considerations are the availability of water all year around and the chemical composition. External exchangers can be used to protect the heat pump and increase life expectency but this will reduce the kw output slightly.

4. Air Source

Air source systems require no ground works but utilise the air that we breathe. Because of this the systems are less expensive and are more suitable for private developments or social housing.

Energy is extracted by an external fan unit that draws the air in at the current temperature. Air source systems are more susceptible to exterior air changes and as such are not as efficient as ground source in extreme conditions but conversely they react to increased temperature changes during spring and autumn seasons.

To see our Air Source Heat Pumps please follow the links below

IDM - Terra CL

Daikin - Altherma Split

Daikin - Altherma Monobloc

Daikin - Altherma HT

To Download An Introduction To Heat Pumps Please Click Here