By: John B Quinton CSci, CPhys, MInstP BSc
Before commenting on practical costs of particular kinds of renewable energy systems it is necessary to sketch the contexts within which they are, firstly, normally compared with conventional sources and then compared with each other. And their technology specific limitations, which determine the kinds of applications in which they are most useful, are also relevant.
So, firstly, whether or not it makes sense to switch to a degree of dependence on renewable energy depends on two cost factors - how much grant is available for a renewable source that will make a significant contribution to a user's total energy need, and the current price of the alternative, (i.e. the conventional, public source). As prices of electricity, gas or oil double, the length of any pay-back period for a system that replaces it halves. If price doubles again payback halves again. But the effect of a subsidy, (grant), affects only the capital cost of equipment and it bears no relationship to the practicality* or efficiency of the system of which it forms part.
Decision-making is confused by another factor. Most installations will have at least a 25 year life, yet all of the effective technologies are subject to intensive development which will affect both their future capital cost and also their efficiency - their ability to take the maximum benefit from the basic energy source, which is the sun. The energy available from the sun during daylight varies across the UK, but is generally the equivalent of 1,000 watts per square metre of surface area. Solar thermal energy in the form of directly heated water is already well developed, and might extract as much as 600 watts, (60% efficiency), whereas photo-voltaic, (PV), cells, which generate electricity, and which many believe will eventually*** be the technology of choice, (producing DC), are presently only around 10/12% efficient, (100 watts per sq. m.); but are subject to intense development.
There is one renewable energy source, available only at a minority of sites, which sets a high benchmark for performance, and that is hydropower. The simplest of technologies - a simple wheel driven by a water flow, (a mill wheel) - offers 90% energy conversion from flow to rotation, though the necessary use of an inverter, (as with wind-turbines or PV systems), to produce a mains compatible supply will reduce that. But unlike most other forms of conversion this source is available 24 hours a day, (only heat pumps, which are not in the purest sense renewable energy devices, have that capacity.
Finally, most renewable energy is produced only when the sun shines or the wind blows. Only heat pumps and systems powered by water-flow are truly 24/7 technologies. All other systems need energy stores if energy is needed at night or in calm conditions - heat stores for thermal systems and batteries (or the National Grid) for electrical systems.
Practical Renewable Energy Systems
It should be stressed again that all renewable energy systems are very site, use and management sensitive. Though the following figures give a general guide as to costs and pay-back for particular technologies, a site specific survey would be required to establish the likely performance of a particular technology, or mix of technologies, when the principal concern is cost and payback. It is often forgotten that when payback has been achieved the energy generated is then free for the life of the equipment used.
Solar Thermal systems, using roof-mounted panels that use the sun's energy to heat water directly. Wild claims are made by different manufacturers as to their efficacy, but in practice it is likely that an optimised and properly managed system will contribute up to 50% of the total heat energy needs of a domestic dwelling in a year. With no grant available and a mean cost of around £3,500 + 5% VAT, that should give a payback in 6/7 years, greatly reduced if a substantial grant** is available.
PV systems are currently too expensive*** to justify dependence on them except where there is no alternative energy source, such as for remote homes, agricultural or fishery activities where a continuous supply is not essential and where cost would preclude a public supply. There are multi-ply roofing systems available where the additional cost of building in PV capable layers is marginal, and which find applications in commercial and industrial new builds or major refurbishment where premium values, and so costs, can be justified. Payback at present rates and efficiencies, (see above), are likely to be around 25 years at today's prices. Some funding sources, government and local authority and those concerned with prestigious projects, consider that to be acceptable.
The larger a wind-turbine the cheaper its output in terms of kWh per pound of capital cost. Very small wind turbines can be justified only when they are the only practical source of energy. In a domestic application a WT rated at 6kW will, in practice, produce about a quarter of its possible maximum - about 1mWh - saving around £1,000 a year, (at 10p a unit), for a 20 year payback if no grant is available. Much of the energy is likely to be produced when it is not needed locally, so such systems are generally connected to the National Grid, (Grid connected). which will pay for any energy supplied, though at less than it would charge to supply it, (but see *** regarding, feed-in tariffs).
Heat Pumps operate round the clock and regardless of the weather. They effectively recover energy from the sun stored in the surface layer, (or in a variant, using borehole for a geo-thermal effect), when configured as ground source heat pumps****. They consume electrical energy, which drives the heat pump, and their effectiveness is measured by the energy multiplication factor they achieve - typically 4 or 5 times. The energy gain for a 2 kW heat pump should be around 160 kWh daily providing, free energy worth around £1,200 a year at current prices, with a pay-back of around 12 years - at current energy prices, but halving to 6 as electricity/oil prices double. (Crude oil prices trebled in one year in 2006, and having fallen back since then, are now increasing rapidly again.
Micro-hydro systems develop electricity using either streambed axial flow turbines or flow driven wheels - waterwheels. These also work round the clock and regardless of the weather. An existing mill wheel converted to drive a generator is likely to have a power of at least 6kW, which is considerably more than the average domestic consumption. It is therefore capable of providing an independent and sufficient source with a surplus for export to the Grid. Depending on whether the facility is provided as a refurbishment of an existing mill or as a new build payback can be as short as 4 years.
* Grants are offered for political reasons, under the LCBP and otherwise. They become available and can be increased, reduced and cancelled at random. And they are. Whilst they exist they are subject to budget caps, often very small ones, and to post code limitations. Grants in Scotland, for example, are presently much more generous than they are in England and Wales. But they also vary as to user type - grants available for a particular technology in a domestic application are not necessarily available in a similar commercial and industrial application and vice versa.
** Reducing the payback for such a system in Scotland to around 4 years.
*** Feed-in tariffs - large payments for electricity produced locally - are already paid in some countries, and are due to be introduced for the UK in 2011. Planning regulations will increasingly be used to secure all forms of local energy generation, building on current regulations.
**** There are cheaper, though less efficient, air source heat pumps and, less commonly, water source heat pumps.
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