After space and water heating, electrical power places the biggest burden on a building’s energy consumption. In the home, electricity powers appliances, cooking, lighting, fans and pumps. Currently, most electricity is centrally generated using fossil fuels and distributed through the national grid resulting in energy use that is both inefficient and environmentally damaging.
The climate change agenda is forcing change. Along with the key UK government policy to reduce the amount of power produced using fossil fuels, other policies are driving production to the smaller scale near and on-site generation technologies, aka ‘Distributed Generation’ (see also ‘Electricity: towards ‘Distributed Generation’).
At a domestic level it is becoming increasingly possible to reduce or, in some cases, eliminate the dependence on grid electricity. Much of the reduction can be made by the more efficient use of power in the home, whether from the replacement of inefficient appliances or a more disciplined use of electricity, but the more significant impact will be made from the installation of communal and dwelling-specific renewable electricity generators aka ‘Microgeneration’.
Unlike the renewable energy technologies employed to produce heat such as solar hot water and heat pumps, the production of electrical power can be more complex and demanding on capital investment, technological sophistication and planning. Because economies of scale are likely to be involved, domestic-scale solutions will not always be the best. Often larger communal or dedicated off-site generation will be favoured over equipment installed in individual dwellings. Before designing to install renewable power technologies as part of a refurbishment project, consideration should be made to the implications and possibilities of planning at a larger scale.
Together with wind turbines, PV is the most ‘tried and tested’ generation technology – and arguably the more suitable at a domestic scale. Initially conceived to power orbiting satellites in the 1960s, PV technology has continued to develop through into the 21st century. Once the sole province of NASA and wealthy earlier-adopters, PV cell manufacturers have, over the last few years, expanded into the mass-market. Development of more efficient materials and process combined with the economies of scale will ensure increasing popularity through a reduction in unit price and greater energy deliverability.
Renewable Energy generated
PV modules convert sunlight into a direct current (for a more in-depth explanation of PV technology, see ‘Photo Voltaic (PV) cells’. Depending on the particular PV technology installed, a typical 1m2 module will generate anywhere between 60 – 110 kWh per year and a typical maximum efficiency will be 40%. The immediate output will be as DC - so for use in most domestic applications, an inverter will be required to convert the current to AC.
Orientation and inclination
For optimum output, a panel should be installed on a roof facing between South East and South West and be unobstructed by shade throughout the day.
For optimum output, the tilt from the horizontal will be equal to the latitude of the site minus approximately 20 degrees. For example 30 degrees is an optimal tilt in Southern England, increasing to almost 40 degrees in Northern Scotland.
Alternatives to pitched roofs
If an installation to a roof in unfeasible, a wall mounted vertical array will produce around 70% of maximum output. Another alternative might be to install the array on a flat roof using a proprietary A-frame mounting device.
Types of PV module
PV modules are available as homogeneous panels, roof tiles or, more rarely, as film for integration with glazing.
Standard size panels can be linked to provide an array. A typical domestic array will comprise of up to 8 panels, depending on manufacture. Mounting the panels on a roof will use aluminium or stainless steel components provided by the supplier. There are two major types of mounting: (1) roof integrated where the panels replace the tile and become flush with the rest of the roof or (2) on-top mounting where the solar power panels sit above existing roof tiles/slates.
Roof integrated PV tiles are available from a range of manufacturers. This format of module might be considered as part of an overall roof covering replacement. Tiles are supplied pre-wired with connectectors to adjacent tiles.
The size of a solar PV system is referred to in terms of power output in full sunlight, known as its kilowatt peak (kWp), and is usually governed by the available roof area and budget. A typical 2 kWp system, occupying around 20m2 of roof space, would generate between 1500-1800 kWh of electricity per annum, which can be up to 50% of the consumption of an average household (assuming an annual consumption of 3300kWp per year). A typical capital cost would be around £500 per square metre. See also: ‘Durability – Photovoltaics’ for more information about specification options.
PV systems can be designed to export and sell any surplus solar power energy back to electricity grid rather than store the energy. This method, by using the grid as a kind of battery, overcomes many of the shortcomings of using regular batteries; including high cost, storage, limited useful lifespan and environmentally unfriendly components.
• Desired output calculated from expected overall household demand.
• The extent of available roof space.
• Extra weight on roof structure.
• Factors that determine pitch, orientation and shading.
• Mains connection and ‘export’ facilities.
• Planning issues – eg historic buildings, conservation areas etc.
There has been a sizeable backlash in recent times to the installation of small domestic wind turbines. In almost all urban areas where they are commonly ‘bolted-on’ to buildings (almost never a good idea) they are unsuitable, likely at most times to produce only enough energy to power a light bulb.
Small wind turbines usually need wind speeds of at least 5m/s, but in urban areas, the actual wind speed at roof level is likely to be under 2m/s at rooftop height and turbulent in quality.
Wind conditions in rural areas will probably be more suitable. An ideal location will be one where the wind flow is unobstructed and where the speed averages in excess of 5m/s. In such conditions, a typical 1kW turbine might be expected to achieve around 3,000 kWh per year.
Economies of scale are particularly appropriate when considering wind power. A group of dwellings or a neighbourhood might invest in a medium / large-scale turbine to work at an appropriate height, where the electricity generated is exported directly to the grid (a ‘community wind turbine’ or CWT). Electricity is drawn from the grid by households in the normal way, but the income from selling power offsets the cost of the electricity bought from the grid.
Private wire supply
Alternatively a group of dwellings can be connected to an ‘on-site’ electricity ‘node’. The node can be connected in turn to any number of generation devices whether they be CHP (see below), wind turbine(s), PV or the grid. Available renewable energy will be supplied through the node, with any shortfall being made-up via the grid and bought from the supply company at a bulk-purchase rate. Any excess electricity generated on-site can be sold back to the grid.
For more information about domestic-scale wind turbines, see also ‘Small scale wind turbines’
CHP and Micro-CHP
Combined Heat & Power (CHP) is a process by which space / water heating and electricity are produced at the same time.
Community scale CHP units have been operating since the 1970s but recent technological developments have enabled domestic-scale, ‘Micro-CHP’ units to become available on the market. Currently though, electricity generation inefficiencies have meant that uptake on the basis of CO2 emissions have been limited. Some argue that a typical currently available Micro-CHP unit is no more efficient in terms of carbon output than getting heat from a condensing boiler and electricity from a gas-powered power station.
It is still early days for Micro-CHP, but as newer technologies, such as fuel cells with their higher efficiencies, become more popularly available to power CHPs, it is likely that Micro-CHP will be the leading technology in the production of heat and power.
As a part of future housing refurbishment, Micro-CHP units, being similar in size to conventional boilers, will be relatively easy to install. But currently, the relatively new technologies involved, the high capital cost and low life-expectancy are likely to make the units only suitable for the upper-end of the housing market.
For more information about Micro-CHP, see also ‘Micro-CHP’ and ‘Durability – CHP’
For more information about fuel cells, see also ‘Fuel cells’
• Solar Technologies for Buildings, Ursula Eicker, 2003
• Understanding building photovoltaics, CIBSE, 2000
• Wind Energy - The Facts: A guide to the technology etc., European Wind Energy Assoc. 2009
• Wind Turbines: Fundamentals, Technologies etc., Erich Hau, 2005
• Wind energy basics: A guide to small and micro wind turbines, Paul Gipe, 1999
• Micro-Wind Turbines in Urban Environments, Phillips et al, BRE Press, 2007
• CHP for Existing Buildings - Guidance on design and installation, Teekaram et al, 2007
• Small-scale combined heat and power, CIBSE 1999
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