The different microgeneration technologies

The official definition of microgeneration in the 2004 Energy Act is the generation of energy up to 45 kiloWatts (kW) of heat or up to 50 kW of electricity. For the average home you are unlikely to install more than 15 kW of capacity, be it heat or power, as any more would be an expensive choice.  Microgeneration is not restricted to renewable energy and also covers low and zero carbon technologies. At the Good Energy Shop we are aiming to offer five types of microgeneration. We are currently offering solar thermal, solar photo voltaic, micro wind and heat pumps and in the future we will have biomass heating.

Solar thermal hot water

This uses the radiation from the sun to heat your water, working alongside your existing water heater. It can be used in the home or for larger applications such as swimming pools. For indoor hot water there are three main components: flat bed panels/evacuated tubes, a heat transfer system and a hot water cylinder. The panels or evacuated tubes 'collect' heat in a mixture of water and anti-freeze (usually glycol) which is then used to heat water by the heat transfer system. The hot water is then stored in the cylinder that can be used instantly or later in the day.

Evacuated tubes vs. flat bed panels

Flat bed panels are a thin panel of metal that acts as an absorber positioned just below a sheet of glass. These are robust, long lasting systems. Evacuated tubes have thin strips of twisted metal - which again act as an absorber - positioned in  vacuum filled tubes. The vacuum means conduction losses are reduced so evacuated tubes are typically more efficient whilst flat bed panels tend to lose more heat, especially when it's cold.

The evacuated tube system consists of a row of these tubes and is better in climates where there is not always direct sunlight i.e. when the radiation is high but not necessarily the temperature. However the increased efficiency of evacuated tubes does not necessarily mean that they can produce more hot water just that the area can be reduced.

Performance

A typical domestic solar hot water system can provide for almost all of an average family's hot water in the summer months and about 50 - 70% of annual requirement, saving about 0.2 to 0.4 tonnes of CO2 per year, depending on the fuel replaced. (Source: micropower).

Requirements

You will need 3 to 4 metre squared area of southeast to southwest facing roof that receives direct sunlight for most of the day. Completely south-facing is preferred. You may also need the space for an additional water cylinder.

Maintenance

There should be few maintenance costs as long as the panels/collectors are kept clean and out of shade. The wiring and components of the system should be checked visually by a qualified technician at one year and then again at five years. At ten years the glycol in the heat collectors will need changing which will take a couple of hours. 

Advantages

• Most systems provide all hot water requirements during the summer months and 50% - 70% of needs averaged over the whole year.
• Doesn't require planning permission unless in a listed building, conservation area or an area of natural beauty. However we recommend that you check with your local planning authority right at the start of your project. See the Energy Savings Trust planning section

Disadvantages

• Normally not compatible with a gas fired combination boiler
• Effectiveness can be reduced in winter and by shading 

Wind turbines

The UK has an abundance of good-quality wind.  In fact it is estimated we have 40% of Europe's wind resource. In the modern wind turbine, the wind turns the blades that drive a generator that creates direct current electricity. The quantity of power produced is dependent on two factors: wind speed and rotor diameter. It is proportional to the cube of the wind's speed, which means that a small increase in speed will result in a large increase in potential output.

Performance

The performance of a wind turbine is highly dependent on the level and constancy of the wind speed and the quality of the wind (i.e free from turbulence) at your chosen site.

To illustrate the importance of wind resource we looked at three customers on Good Energy's Home Generation scheme. Each customer has installed a 2.5kW wind turbine but live in areas with differing wind resource. The second column in this table is the capacity factor which is another way of stating the annual energy output from a wind turbine in its given location. The capacity factor is the actual annual energy output divided by the theoretical maximum output, if the machine were running at its rated (maximum) power during all of the 8766 hours of the year. For example: If a 600 kW turbine produces 1.5 million kWh in a year, its capacity factor is = 1500000 / ( 8766 hours x 600 kW ) = 1500000 / 5259600 = 0.285 = 28.5 %.  Capacity factors may theoretically vary from 0 to 100 per cent, but in practice they will mostly be around 15-30 per cent.

This table clearly shows the vast difference in productivity of a wind turbine sited correctly to one that isn't. However depending on the requirements of your home a well-sited 2.5 kW turbine could produce up to a half to two-thirds of your electricity needs (based on an average 3300 kWh home).

Requirements

Generating electricity with a wind turbine requires a strong clean wind resource - because it is so dependent on wind speed and direction it's important to make sure you are in the right location to make a turbine viable. If generating electricity is your main motivation it's suggested that you only consider a turbine if your local annual average wind speed is 6m/s or more. An approximate figure for your area can be checked on the BWEA (British Wind Energy Association) website.

Ideally your turbine should be high up (about 9 metres above other obstructions) and with no nearby obstructions such as buildings, trees or hills that could reduce wind speed or increase turbulence. This means no obstacles for about 50 metres. We insist that you have a professional assessment done before going ahead with your microgeneration project.

Maintenance

Wind turbines do not require a large amount of maintenance. Turbines can have a life of up to twenty years but they do require service checks every few years.

Advantages

• A wide range of wind turbines are available to suit your needs and your location
• Long operating life and low maintenance requirements

Disadvantages

• Usually requires planning permission and you must check the policy of your local authority. See the Energy Savings Trust planning section
• Only viable if you have sufficient wind speed and are in an appropriate location

Heat pumps

These work by using the heat from the sun that is stored in the earth, water or air. This is then used to heat a building and, in some cases, to heat hot water.

For a ground source heat pump lengths of pipe are buried in the ground and filled with a mixture of water and antifreeze. The liquid is pumped around the pipe and, as it moves around the loop, it absorbs heat. For a lake/river source heat pump the water is drawn up directly to the pump's heat exchanger where its heat is extracted and the water is returned to the source. The process of heat extraction for both pumps is similar to a fridge except it runs in reverse: you have an evaporator that takes the heat from the water or water/anti-freeze and a condenser that gives up the heat to a hot water tank. The heat can then be distributed via under floor heating or radiators or, in some cases, for hot water supply.

Air source heat pumps absorb heat from the outside to heat buildings. There are two types of air-source heating systems: air-to-air systems provide warm air to heat the building and air-to-water heat water to provide heating through radiators or an underfloor system. Air Source Heat Pumps draw air across the water-anti freeze solution and so the heat energy is transferred into the refrigerant. It then uses the 'backward-fridge' process similar to ground and lake source pumps. However unlike a ground source pump the system uses no external pipes and most of the working elements reside within the building.

Performance

The pumps require electricity to work and the performance of heat pumps is usually measured by a coefficient of performance (CoP). This is the ratio of units of heat output for each unit of electricity used to power the heat pump. Typical CoPs range from 2.5 to 4. So for a CoP of 4 for every unit of electricity you input you are returned with up to 4 units of heat.

Based on current fuel prices a ground source heat pump with a CoP of 3 - 4 is cheaper than heating fuelled by oil, LPG and electric storage heaters (source: micropower). They can provide 100% heating and hot water but running costs vary depending on the heating system and insulation of house. Generally in a well insulated house, heating costs compared to fossil fuels can be between 40 to 60% savings. 

Requirements

For a ground system you will need adequate space and a suitable area to dig a trench or borehole for the pipe. We estimate a minimum of 20 m x 5m for a ground system. If you are using trenches they must be about 1.5m deep and roughly 10 m of trench is required to generate 1 kW of output. If you're building a new house or renovating your garden it could be a perfect time to install a ground system.

For a lake system you need a body of water with similar dimensions that are required for ground source, namely 20m x 5m and a depth of at least 1.5m. For an air source system you will need room on the outside of the house to install a pump.

Heat pumps work best in a well insulated house and are particularly good for under floor heating. We recommend that they are installed in houses built after 2000 and, optimally, with underfloor heating. It's also worth bearing in mind that you will need power for the compressor and the pump which will have to come from other sources such as standard electricity or a solar PV. However if it's your CO2 emissions you're worried about then if you're signed up to Good Energy it's not a problem!

Maintenance

Once installed, heat source pumps should be checked about once a year by an expert to make sure the filters and coils are clean otherwise their performance will be diminished.

Advantages

• Heat pumps tap into a constant heat source rarely affected by surface temperatures, offering year-round heating.

• Don't require planning permission unless in a listed building, conservation area or an area of natural beauty. However we recommend that you check with your local planning authority right at the start of your project. See the Energy Savings Trust planning section

• Heat pumps can be combined with solar thermal heating to provide a complete hot water and heating solution.

Disadvantages

• Need for mains electricity for operation
• For ground source pumps you require sufficient space as the system sizes are quite big

Solar Photo Voltaic (PV)

This technology uses photovoltaic (PV) cells to convert radiation from the sun into electricity.  A typical PV cell consists of a wafer of semi-conducting material, usually silicon, manufactured with two electrically different layers.  When sunlight hits the cell it excites the electrons within the silicon, creating an electric field across the layers and causing a flow of electricity.  

There are several types of PV cell. Monocrystalline cells are made from a single large crystal of silicon - they are more efficient and slightly better in low light conditions, but they can be more expensive. Polycrystalline cells are made from cast blocks of silicon that contain many small crystals and are slightly less efficient than monocrystalline cells. In practice, for a typical residential property, there is little difference in the performance of these different products.

A solar PV panel (or module) is made up of a series of cells and the greater the light intensity on the panels the more electricity will be produced.  A typical panel used in a residential PV system will have the capacity to generate between 160 and 190Wp (Watt-peak). A number of PV panels can be linked together to make a PV array.  So, for example, a system consisting of 10 x 180Wp panels would have a peak output of 1,800Wp - or 1.8kWp (kiloWatt peak).

The Good Energy Shop solar PV panels are designed to be mounted above an existing or new roof. They are suitable for almost all types of roofs such as slates, concrete or clay tiles. The systems will be installed on behalf of Good Energy by Sundog Energy Ltd, one of the UK's leading solar PV design and installation companies.

Operation

A PV array generates direct current (DC) electricity. However all PV systems from the Good Energy shop come with an inverter which converts the DC to AC electricity and synchronises this to match the electricity you normally get from the grid. It is connected directly into the electricity distribution system in your home so you can use your solar electricity just as you use your existing mains supply to power all your lighting and electric appliances. There are no batteries or other complicated pieces of equipment to worry about. The only thing you will notice will be a reduction in your electricity bills!

Any surplus electricity you generate will be exported through the mains. If you buy your electricity from Good Energy we will pay you for your excess electricity through our award winning HomeGen scheme which pays 15p for every unit (kW) of electricity generated - even the units you use on site. Our research shows this is the highest paying reward offered by any supplier in the UK.*

Output & Money Savings

The national average household consumption of electricity (excluding heating) is 3,300kWh. A 2kWp solar PV system in a good location in the UK will generate around 1,700kWh of electricity per year. So this is enough to meet just over half of the average household's electricity needs. This system would also save almost a tonne of CO₂ a year.

If half of the electricity generated by the PV system is used on site and half is exported, the annual saving would be £305* p.a. (this includes the 15p per unit paid under the Good Energy HomeGen scheme**)

(* based on Good Energy tariff of 15.9p per kWh)

Requirements

For a solar PV system to work optimally the mounting site should face south and there should be no other buildings or trees overshadowing it. If it is facing south-east or south-west it will produce about 90 - 95% of the optimum and those facing east or west will produce about 80 - 85% (depending on roof pitch). Approximately 8 square metres of PV modules are required to generate 1kWp. So a 2kWp system will require at least 16 square metres. In practice a slightly larger roof area than this will be required in order to provide a protection and access zone around the array.

Maintenance

Solar PV panels have no moving parts and require no maintenance. Normal rainfall is usually sufficient to keep the modules clean. The photovoltaic modules we supply through the Good Energy Shop are made in the UK by Sharp, are very reliable and come with a 25 year performance guarantee.

Advantages

• The size of system is adjustable depending on how many panels are used.
• Solar PV does not require planning permission unless on a listed building, or in a conservation area, area of outstanding natural beauty (AONB) or a National Park. However we recommend that you check with your local planning authority before you start your project. See Energy Saving Trust Planning section
• There is no regular maintenance requirement
• Once installed all the power produced is totally cost - and carbon - free. There is no need for any external pump or fuel source to drive the system
• A solar PV system can add value to your property so it is likely that you will recoup the cost of a system if and when you sell your property

Disadvantages

• Relatively high initial cost (although government grants of up to £2,500 are currently available for domestic systems)
• Generation does not necessarily coincide with peak demand i.e. it is likely to be during the day when most of the household may be out.  This is not such a problem for those working from home or looking after the family, however, you can always install timers to schedule your washing machine, dishwasher and other appliances to come on during this time.

Biomass Heating

Biomass heating involves the burning of organic material. For household microgeneration biomass most often takes the form of wood pellets, wood chips and wood logs. The CO2 released when energy is generated from biomass is balanced by that absorbed during its production. Depending on the transport of fuel and other carbon overheads this can make it virtually carbon neutral. It's commonly asked whether burning wood is a 'climate friendly' energy source but if you consider that a tree left to rot produces just as much CO2 as burning it, then yes it is.

For centuries man has been drawing heat from biomass and these days modern technologies have been developed to be highly efficient. Two commonly used wood heating systems are stand-alone stoves and boilers connected to central heating and hot water systems.

Performance

The performance of biomass heating depends upon the chosen system, how much the system is used and which fuel you are replacing. The stand-alone stoves for space heating have a typical output of about 6 - 12 kW. The boilers that are connected to central heating and hot water systems are larger with an output of more than 15 kW. (Source: micropower)

The cost of heating with wood pellets is 3p/kWh compared to heating oil at approximately 6p/kWh and electricity which is 12p/kWh. (Source: Forestry Commission)

A biomass stove could provide a detached home with 10% of annual space heating requirements and save around 840kg of carbon dioxide when installed in an electrically heated home. A wood pellet boiler could save you around 6 tonnes of C0₂ per year when installed in an electrically heated home. (Source: EST)

Requirements

For either stoves or boilers you will need storage space for fuel and a flue with sufficient air movement for the stove to work effectively. You will also need to ensure that the installation complies with all safety and building regulations and that you aren't in a smokeless zone. The Good Energy Shop are currently looking into supplying fuel which they hope to do soon on the site.

Maintenance

Like any wood-burning device it must be kept clean, especially the flue, and the ash will have to be removed.

Advantages

• Virtually carbon neutral due to closed carbon cycle
• Potential to support local fuel companies

Disadvantages

• Storage space is required  for the volume of fuel, especially wood chips or logs. Less space is required for pellets.