With sustainability high on the agenda of many self builders, it is important to know what types of renewable energy technology are available to the homeowner.

Here, Tim Pullen and David Hilton look at different energy sources and their applications, including costs and suitability.

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Solar Photovoltaic

Solar PV systems convert sunlight directly into energy and are the most common form of renewable energy system.

PV will not give you all of the energy you require in your home without storage batteries, and the Feed-in-Tariff (FiT) payment is not as good as it used to be. But the installation costs are much less than they were, and when you plan your lifestyle and energy consumption to maximise the use of the generated electricity it is still a good option.

A system of around 4kW generation capacity (under optimum sunshine) is the most common, as installers can install and self-certify an array of this size without special permission from the distribution network operator.

Check out the supply company: obtain references, follow them up and make sure the company has a good track record. And do NOT lease your roof space to someone else. They will make the money and you will not.

Is Solar PV Right for My Property?

  • Solar PV will suit a higher proportion of properties than other renewable technologies and has a long and largely maintenance-free life.
  • Planning consent is not usually needed but it is wise to check.
  • A solar array does not need to be mounted on the roof but it needs to be as close to south-facing as possible (within 45°) and inclined as close to 30° as possible.
  • The array needs to be free from shadows (including those cast by trees and neighbouring houses) and not likely to be hit by cricket balls or other hard objects.
  • PV systems are fairly low-maintenance products and periodic visual inspections of the wiring and panel integrity, as well as maintaining the cleanliness of the panels, is all that’s required. Many inverters now have apps that monitor the generation and will quickly flag up any irregularities.

How Much Does Solar PV Cost?

The unit cost reduces with the increasing size of the system. This is because the cost of the other elements – inverter, control gear, scaffolding and so on – remains largely the same. This makes schemes below 2kW difficult to justify in financial terms.

For a 4kW array, expect to pay:

  • £6,000-£7,000 for the installed cost of on-roof panels
  • £7,000-£8,000 for in-roof panels (installed flush with the roof cladding and usually replacing the tiles)
  • £8,000-£11,000 for more discreet slate-type individual panel systems.

Allow an additional £600-£1,000 to replace the inverter unit after 10+ years.

The payback period for a PV system at current electricity prices is usually around £500-£600 per year. You could therefore expect the £6,000 system to be paid back in 10-12 years, leaving you with 8-10 years of free energy with profit.


Biomass

Biomass heating is the combustion of grown products, normally wood based: wood pellets, logs and chips. Pellets are clean, easy and expensive. Logs are cheap, more messy and more work. Woodchip is generally for big – 50kW plus – boilers, it’s messy and it needs a lot of space.

Both pellet and log machines are available as boilers or stoves with back boilers. The main difference is that pellet stoves/boilers work as a principal heat source but log stoves/boilers don’t. This is because pellets have a standard calorific value and are fed in a slow continuous trickle to the burner, which means that a given level of heat output can be maintained. Logs are thrown into the stove willy-nilly and heat output will vary with the quality of the logs and the amount of wood in the stove.

A gasifying log batch boiler works well as a principal heat source for houses that need a reasonably high level of heat, but they do require logs to be loaded manually. Wood pellet boilers are far more automated but the equipment is more expensive, as is the fuel.

Efficiency is generally up to 90% and biomass boilers tend to have a long life of 20-plus years.

Is Biomass Right for My Property?

  • Biomass boilers are generally much larger than conventional boilers, so you will need plenty of space to house them, along with a store for pellets, logs or chips. For example, a one tonne palette of bagged pellets will take up roughly the same floor areas as the boiler.
  • Biomass makes more sense in larger homes with a higher heat demand, where renewable heat incentive (RHI) payments are going to be able to offset the capital cost of buying and installing the boiler.

How Much Does Biomass Cost?

  • The cost of log batch boilers is around £5,000-£10,000 depending on the capacity and quality. The cost can also vary depending on the capacity and quality of the thermal stores and the complexity of the integration of the system with the home.
  • The cost of installing pellet boilers starts from around £6,000-£12,000 depending on the quality of the boiler and the complexity of the flue and pellet store. Often the cheaper boilers have a higher ash tolerance so they are able to burn lower quality pellets, but you will then also have lower efficiency and higher emissions.

Solar Thermal

Solar thermal systems use the heat from the sun to heat your hot water. Solar thermal systems are robust, effective, have a long life and are relatively cheap to install.

There are two types: flat plate and evacuated tube:

  • Flat plate systems are cheaper to install and evacuated tube systems are slightly more efficient. If the roof is close to due south then flat plate will be as good, and cheaper, than evacuated tube.
  • Solar evacuated tubes are very useful for difficult installations. An evacuated tube system can be installed by a single fitter as there is no heavy lifting involved.

In most cases the solar thermal system does not contribute to the central heating, as there is less solar resource during the heating season and so it would not be of much benefit. However there have been new developments in thermal store and cylinder design. Under some circumstances, especially where a high temperature boiler back up is used, it may be beneficial to use the low-grade heat in the winter months to aid central heating rather than focus solely on domestic hot water.

With the right hot water storage tank and control systems running underfloor heating, there is also a good argument for increasing the size of the solar thermal array by 50%. The array will then make a larger contribution to space heating and allow the boiler to shut down from late spring to early autumn, rather than just the summer.

Is Solar Thermal Right for My Property?

  • Solar thermal systems don’t work with most combi boilers, as they need a large hot water storage tank to store the energy produced.
  • Solar thermal can typically be installed under Permitted Development (i.e. without planning) except in sensitive areas, such as Conservation Areas or near listed buildings.

How Much Does Solar Thermal Cost?

A solar thermal system installation will cost from around £5,000 for a two panel flat plate system and around £6,000 for a similar capacity evacuated tube system.


Heat Pumps (Air & Ground)

Next to solar thermal, heat pumps are the most commonly used technology. Effectively a fridge in reverse, the heat pump uses the refrigeration cycle to move heat from the air or the ground and transfers it as useable heat in your home. These units physically replace the boiler, but as they deliver lower temperatures than a boiler, they need to be carefully designed into well insulated, airtight homes for optimum efficiency.

There are two types of heat pump technology:

  • Air source heat pumps extract heat from the air and runs it through a heat exchanger. The unit looks similar to an external air conditioning kit.
  • Ground source pipes extract latent heat from buried ground collectors, such as trench based ‘slinkies’ or more expensive boreholes.

Both types of heat pump use an efficiency measurement known as the Seasonal Performance Factor (SPF). This indicates performance over a whole heating season, within set parameters. The Ofgem database uses a Seasonal COP or SCOP, but the calculation is the same.

An advertised SPF will be based on a flow temperature of perhaps 35°C, which suitable for underfloor heating but not radiators or domestic hot water. If the heat pump is used for anything other than underfloor heating, the SPF will fall.

In the case of air-source heat pumps, the SPF is also affected by the outside air temperature, so it’s lower in winter when the most heat is needed. There is evidence that some air source heat pumps, used on their own, emit more CO2 and cost more to run than an equivalent gas boiler.

Are Heat Pumps Right for My Property?

  • You need a well-insulated, relatively airtight house and a supplementary heat source. Ideally this will be a solar thermal array, but a simple immersion heater running on dual-tariff electricity is a cost-effective alternative.
  • For ground source heat pumps, you need a reasonably large garden or plot of land for the buried ground collectors, such as slinkies or boreholes.
  • Make sure that your heat pump is sized correctly, and that the house is designed with a heat pump in mind. You may want to use an energy consultation to advise you.

 How Much do Heat Pumps Cost?

  • The cost of installing a ground source heat pump is around £1,500-£2,000 per
    kilowatt.
  • An air source heat pump will cost around £5,000 as the base cost, and then an additional £500 per kilowatt.

Wind Turbines

Wind turbines convert the wind’s kinetic energy into mechanical energy. While less popular than other renewable options, a small 1kW-2kW wind turbine may still be a viable option for some self builders. That solution might be improved by adding battery storage to the system, like the Sunamp or Tesla system.

However, the economic case for installing a 5kW or 10kW domestic scale wind turbine (based on installation costs and current tariffs) no longer stacks up.

Is a Wind Turbine Right for My Property?

  • Wind speed and lack of turbulence are key. Generally the annual average wind speed needs to be over 5 metres per second (m/s)
  • The turbine needs to be sited away from buildings, trees and so on.
  • It needs to not cause ‘offence’ to neighbours (typically noise offence), listed buildings or national parks
  • Planning consent is needed for wind turbines (but not the small 1kW turbines) and the cost of obtaining it needs to be factored in, as well as the cost of connecting the turbine to the Grid. This can vary from hundreds to tens of thousands of pounds depending on the location and size of the turbine. Get a quote from your Grid operator before making any commitment. Find your local Grid operator at nationalgrid.com.
  • In some areas a bird or bat survey may be needed, or an environmental impact assessment. Check the requirements with the local authority and get prices before committing.

How Much Does a Wind Turbine Cost?

  • £2,000 for a 1kW wind turbine
  • £4,000 for a 2kW system
  • Around £30,00-£40,000 for a 5kW system (including site preparation, planning permission, installation and the system itself)

Hydro Power

Micro hydro technology is available that lets the individual homeowner (with a stream) generate their own power. A hydro turbine of just 500W will produce enough electricity through the year to meet the annual consumption of an energy-efficient home. You could pick up and carry a turbine that size with one hand.

What’s more, pound for pound, it’s the most efficient of all the renewable technologies. A total life in excess of 40 or 50 years, with little annual cost, is not unreasonable.

Is Hydro Power Right for My Property?

  • You will need a stream or river nearby that is large enough, measured by ‘head’ – the vertical distance between the highest and lowest points of the stream and ‘flow’, which is the amount of water passing a point, measured in litres per second.
  • Low head schemes (under 5m head) are less productive and more expensive than medium to high head schemes, and are often uneconomic. A stream with 10m head still needs a good flow rate, but is always worth investigating.
  • Whether the stream is big enough or not can only be established with a site survey. That may cost a few hundred pounds, but could be a very worthwhile investment.

How Much Does a Hydro Power System Cost?

Each scheme is specific to the stream and the price can vary hugely.


Combined Heat & Power

These technologies produces electricity as a by-product of heat, with engines (internal or external combustion) driving a generator. There are a few machines for big applications needing lots of heat but only one for the typical domestic application — the Baxi Ecogen.

The problem with these machines is that they have to run for a long time each day – upwards of 18 hours – to take full advantage of the Feed-in Tariffs rate. That means they produce a lot of heat.

Is Combined Heat & Power Right for My Property?

In many ways, they are most effective in older homes with higher heat loads and mains gas availability, where they can replace gas boilers.


Photovoltaic Thermal

Photovoltaic thermal (PVT) is the best of the combined technologies. PVT looks like a standard PV array but produces high quantities of hot water as well as electricity. Extracting the heat makes the PV element more efficient, increasing electricity production. When combined with a heat pump it gives good year-round performance.

Is Photovoltaic Thermal Right for My Property?

As with PV, the array needs to be:

  • as close to south facing as possible
  • inclined as close to 30° as possible (the array does not need to be mounted on the roof)
  • free from shadows

Bear in mind the PVT is no longer eligible for FiTs or RHI payments. PVT is for people who want to invest in minimising long-term running costs by producing as much energy as possible on site, at the lowest possible unit cost. What this needs, above all, is a specialist installer with the appropriate expertise.

How Much Does a Photovoltaic Thermal System Cost?

Expect to pay around 1.5 to 2 times the cost of an equivalent rated solar PV system.


For all Systems

  • Start by making your self-build or extension as energy efficient as possible (the ‘fabric first approach’)
  • Check that you understand the technology and get advice from your chosen supplier about the best size and specification for your property
  • Select an installer who is certified under the Microgeneration Certification System (MCS) and uses MSC-certified products
  • Check the latest Renewable Heat Incentive (RHI) or Feed in Tariff (FiT), as rates for these government-funded financial incentives change regularly

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