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Renewable heating sources

SDG Ensure access to affordable, reliable, sustainable and modern energy for all

The different seasons and day and night times lead to variations in the heat and light available and present a challenge to their direct use to provide energy. However, this energy can be stored in various ways – directly as heat in the ground, lakes and rivers providing the heat source for heat pump systems or indirectly by the conversion of light into biomass as with plants and photosynthesis; this biomass is in turn burnt to produce heat or steam to drive electric generators


There are three conversion systems that produce heat

  • Absorption of sunlight to produce hot water – solar water heating
  • Concentrating the low-grade heat present in the ground, air or water – heat pumps
  • Burning biomass like wood or wood pellets – biomass

All of these systems can be incorporated into the home and can produce part or all of the heating requirements. The choice of system will depend upon the type of dwelling, its orientation and its location.

The overall conversion efficiency of any heating system will depend upon the uplift temperature as well as the amount of annual solar radiation. The larger the difference between the inlet and outlet water temperatures or the smaller the radiation intensity, the lower will be the efficiency

It will always be cost effective to reduce the heat demand before considering installing any new heating system. This will ensure that the smallest heating system possible can be installed, which will have a lower initial cost as well as a lower running cost.

Whilst on a sunny day, solar radiation can be directly received, on a cloudy day less radiation is received because it is scattered by the water droplets on the clouds. Typical values are shown in the Figure below.

Renewable heating sources

Do you know anyone who has a renewable heating system in their home?
Which renewable heating source do you think would be suitable for your home?

KITH # Activity Age range
3.3 Suitability of renewable heating sources in your home
 10 – 15  
3.4 Sustainability of heating sources
  8 – 11  

Solar water heating

Solar radiation comprises not only visible light, but also comprises longer wave radiation whose energy can be absorbed by gases, liquids or solids and can then be converted into heat energy. Obvious examples are the earth’s atmosphere which absorbs heat as light passes through it and the earth’s surface. It is the balance between absorption and reflection that enables life to be sustained on this planet. With solar heaters, selective absorber materials are used to absorb the infra red rays to heat water or a ‘solar’ fluid passing through a set of tubes.

The basis of all solar heater systems is the ability of the collector to absorb the infra red portion of sunlight and transfer this heat to water flowing in a tube. The flat plate collector is the most commonly used system and consists of a rectangular box typically 1-2 metres long and 800- 1000 millimetres wide. Small tubes which are attached to a black absorber plate run through the box with water flowing through these tubes which are heated by the absorption of the sunlight’s infra red rays.


  • the only external energy required is to pump the water through the solar collector
  • captures energy from sunlight which otherwise would not be utilised
  • negligible environmental emissions and impact
  • depending upon location. can produce up to 60% of hot water needs in the UK


  • possible visual impact on surroundings
  • less hot water in winter than summer
  • output dependent on weather conditions

Conversion process

conversion process

The ray diagram is illustrated in the Figure. Sunlight passes through the glass plate and onto an absorber plate. Most of the sunlight is absorbed; any that is reflected back towards the glass is in turn reflected by a heat reflective layer on the inside of the glass plate.

There are many flat-plate collector designs but generally all consist of: a flat-plate absorber, which intercepts and absorbs the solar energy; a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber; a heat-transport fluid flowing through tubes to remove heat from the absorber; and a heat insulating backing.


As a typical panel is 2 metres long by 1 metre wide, such panels are easiest mounted on the roof of a building if it is favourably orientated to receive sunlight during most of the day. The collector should ideally be mounted to face south and the optimum angle of mounting is related to the degree of latitude. For a flat roof this is easily achieved using a suitable frame. For a pitched roof, the collector is generally mounted parallel to the roof or for a new build can form part of the roof. If it is possible to adjust the collector angle, then an angle of 30º in summer and an angle of 70º in winter as the sun is lower in the sky are optimal in the UK.


Providing hot water

A typical water heating system is illustrated which shows the solar collector, the hot water storage tank and the associated pipe work. The principal components comprise -

  • solar water heater panel
  • dual coil hot water cylinder
  • water pump
  • temperature controller to ensure that the hot water is available at the appropriate temperature.

The fluid flowing through the solar collector is heated and then passes into the storage tank through a coil which exchanges heat to the incoming cold water. The cooled water then flows back into the solar collector to be reheated. This is an indirect system that allows the working fluid to contain anti-freeze to prevent freezing during very cold weather. In southern Europe where freezing is unlikely to occur, the heated water can be used directly.


To provide a continuous supply of hot water a secondary or auxiliary heating system is required when there is insufficient sunlight during the winter months.

Generally, the plumbing from a solar heater connects to the existing water heater, which stays inactive as long as the water coming in is hot or hotter than the temperature setting on the indoor water heater. When it falls below this temperature, the auxiliary water heater can further heat the water. High-temperature solar water heaters can provide energy-efficient hot water and hot water heat for large commercial and industrial facilities.


During summer there is generally sufficient radiation to provide almost all the hot water demand, whilst during the winter the proportion produced by solar radiation is much less. Overall about 50-60% of the annual hot water demand in the UK can be met.

A typical four-person household uses about 3,000 kWh of energy to provide hot water. This totals about 20% of the annual household use. Because of the diurnal and seasonal variation in sunlight, solar water heaters can generally supply about half of the domestic hot water needs in middle Europe, rather less in northern Europe and about two thirds in southern Europe.

KITH # Activity Age range
6.2 Solar water potential
 9 – 13  

Heat pump heating (and cooling) systems

A heat pump is a system which is able to extract low grade heat from the air, ground or water and concentrate it to provide useful heat for space and water heating. Heat pumps (in spite of their name) can also produce space cooling during the summer. In this process high grade heat is extracted from a building and converted into a lower grade of heat which can then be diffused back into the earth or atmosphere. Thus a single system can produce space heating, space cooling and hot water.


  • Electricity and low grade heat are available from a variety of sources
  • Systems are available in all sizes from heating one room to heating a single or multiple dwellings
  • Some systems are reversible in that they can both heat and cool
  • Electricity to operate the heat pump can come from renewable sources
  • Natural refrigerants are being used which have zero or very low environmental impact


  • Space if a horizontal ground loop collector is used
  • Efficiency drops with decreasing air temperature for air source systems
  • Higher initial costs than conventional heating systems

Energy is only required to concentrate the heat stored directly or indirectly from the sun, not produce it so this type of system has a very high efficiency compared with fossil fuel boilers whose efficiency lies in the range of 0.8 to 0.9.

The ratio of heat out to electricity consumed is called the coefficient of performance and this can vary from 3.0 to 5.0 depending upon –

  • the type of heat pump
  • the difference in temperature between the heat source and the desired output temperature (uplift temperature)

Air source systems
These extract heat from the ambient air and so their efficiency will vary depending upon the outdoor temperature. This poses a problem in very cold weather as the uplift temperature increases and so the overall efficiency will decrease and consequently an auxiliary form of heating may be required.

Ground source systems
These extract heat from the ground via a collector system which transfers heat into a pipe in which a collector fluid flows. The collector pipe may either be laid in a horizontal trench or vertically by inserting into a specially drilled bore hole up to 200m in depth. The drilling of a bore hole requires access for a drilling rig whilst for a horizontal trench, access is needed to a garden or open space. A collector pipe of 100m in length will provide sufficient geothermal heat for an individual heat pump to concentrate the heat and provide between 4 and 6 kW of heat output.

The horizontal trench needs to have sufficient depth, typically 1.2 m in the UK, so that the ground temperature is not influenced to any extent by the ambient air temperature. For northern Europe the trench depth will need to be deeper.

Open loop systems
With these systems, water is extracted from a river, lake, dam or underground aquifer and circulated through a collector pipe to the input side of the evaporator heat exchanger. After transferring heat to the refrigerant, the cooled water is returned to its source.

Heat upgrade process

The heat collected from the source is transferred to the refrigerant in a device called the evaporator heat exchanger which in its simplest form consists of a set of parallel plates. The ducted air or the pumped collector fluid passes through one set of plates and the refrigerant, generally in the form of a low pressure, low temperature liquid, through an adjacent set of plates. As heat always moves from a hotter to a colder medium, the heat is transferred from the source to the refrigerant which is at a lower temperature. This causes the refrigerant to change state from a liquid to a low pressure, low temperature gas.

After leaving the evaporator, the refrigerant is then compressed by the compressor and transformed into a high pressure, high temperature gas. In being compressed, its temperature is raised because the temperature of a fluid rises with increasing pressure.

The refrigerant then passes through the condenser heat exchanger in which the high grade heat of the refrigerant is transferred to the heating medium of the distribution system. If this is air then it can be circulated directly throughout the dwelling using ducting. If this is water then the hot water can be distributed through radiators in the normal way to provide space heating or stored in a tank to supply hot water.

After leaving the condenser, the cooled refrigerant has changed state to a high pressure, low temperature liquid. It then passes through an expansion valve to return the refrigerant to a low pressure, low temperature liquid prior to entering the evaporator heat exchanger and restarting the cycle.

Components of a ground source heat pump system

heat pump

To provide space cooling, the heat pump can extract useful heat from a room and diffuse low grade heat into the air or ground.

Heat pump systems can easily replace fossil fuel boilers, such as oil or electric fired boilers as they are of similar size. Hot water can then either be distributed through a radiator system to provide space heating or to a hot water cylinder to provide hot water.

As is typical of all energy efficient products, heat pumps are characterised by an initial cost which is directly related to the heat output of the system. It is therefore important to size the heat pump according to the heat loss of the dwelling.

For urban settlements, it is important to ensure that the heat loss of the average size building of say 75m2 does not exceed 5.0 kW(h) at -10°C if the geothermal heat present in the underlying aquifer is to be shared equitably between all inhabitants. This upper limit will require additional insulation to be added to the older housing stock in the UK in order to reduce its heat loss.

Since the hot water temperature from a heat pump system will be lower than that of fossil fuel boilers, it is important to improve the insulation level of the dwelling in order to retain the same size of radiators.

In addition it will also be necessary to maintain the thermal efficiency of any radiator system by adding a corrosion inhibitor in order descale the radiators by taking corrosion products plated on the inside of the radiators back into solution. Then to check at the annual service that the water quality is maintained at its neutral level (pH 7.00) by adding additional inhibitor if necessary to ensure limiting corrosion.

One option for locating a GSHP heat pump is in a cupboard with the hot water cylinder mounted above (as illustrated).

Figure 6kW GSHP manufactured by Kensa below with a 150l hot water cylinder above


Reducing the conversion cost

The biggest cost of ground source heat pump systems is the cost of drilling the borehole and so it is always economic to drill an array of boreholes while the drilling machine is on site. The geothermally heated water from these boreholes can then be circulated via a flow and return pipe (ground loop) to a cluster of homes in each of which is located an individual heat pump.

The optimum time to undertake the conversion to low carbon heating systems is when the existing boiler (or electric storage heaters) needs replacing provided that it is possible to access a ground loop in which geothermally heated water is circulated.

Such district energy networks are likely to become more common in future in order to be able to install low carbon forms of heating using ground source heat pumps.

KITH # Activity Age range
7.4 Geothermal heating using heat pumps
  Geography, Science
 12 – 18  

Biomass for heating

Biomass is the oldest and the most commonly used renewable energy source. Biomass is a collective term for plant material that can be burned to produce energy. Examples include wood, straw and energy crops such as willow and poplar.

Solar radiation falling on the earth produces light. This is converted by plants and trees into organic materials by photosynthesis enabling this biomass to grow. The infra red rays associated with sunlight provide suitable conditions for growth such that plants and crops can be harvested in the autumn. Trees take much longer to mature, up to 50 years or more; however short rotation crops can be grown specifically for providing biomass for space heating or hot water. Waste from forestry and farming can also be used.

The most common use of biomass is to replace oil or brown coal boilers.


  • it is a renewable energy source
  • its widespread abundance
  • its general local availability
  • the management of production waste from forestry and farming
  • the generation of local industry and employment


  • requires more space than a conventional gas boiler
  • cannot be used in emission restricted zones
  • pellets need a suitable storage space
  • supply quality can be variable and may not always be available

Useful energy
Biomass can be burned in a conventional boiler that has been adapted for the type of biomass to be used. Generally biomass boilers are somewhat larger because of the lower calorific value of the fuel. This averages 15 MJ/kg compared with 48 MJ/kg for natural gas. It is essential to ensure complete combustion to minimise environmental emissions and residues. Some types of boiler will have automatic feeding of the biomass such as wood pellets, but other types will require supplying fuel at various intervals. The residues are generally beneficial for the soil.

It is generally possible to substitute a biomass boiler for a conventional boiler provided some extra space is available. A flue or chimney is required to disperse the combustion gases and draw fresh air into the boiler. If automatic feeding is required, then space is required for the feed pipe.

A volume of 2 cubic metres is generally required to store sufficient biomass for a period of 3 to 6 months.

Biomass sources include:

  • firewood, logs or wood prepared for burning in a stove or fireplace,
  • wastes and by-products of forest industry, bark, sawdust and shavings, wood chips, trimmings and other tree logging remains,
  • energy crops: the common willow, poplar.

Straw and farming wastes

  • straw from corn, oil plants (rape) and leguminous plants
  • harvest waste, shells from coconuts, remains of corn cobs
  • wastes and by-products of the processing industry, remains after processing sugar cane

Biomass can be processed into briquettes or pellets


Figure Cross section of a typical biomass boiler showing fuel feeding mechanism

KITH # Activity Age range
8.1 Local biomass sources
 9 – 13  


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