(You can download this section as a pdf. A technical version and summary are also available as pdfs to download).

Introduction

It is generally acknowledged that world energy production should come from renewable sources, since the burning of fossil fuels (oil, gas and coal) leads to the production of carbon dioxide (a significant greenhouse gas) and that at the current rate of usage, supplies of fossil fuels are finite and unlikely to be sufficient for all energy needs in the rest of the 21st century.

There are many ways to produce renewable energy, and the United Kingdom has significant resources of wind, tidal and wave energy which potentially can be exploited.

The deployment of wind turbines, particularly onshore, is contentious. As well as comments on the visual and other environmental impacts that wind farms may have, there has recently been comment on the effectiveness of the use of wind as a source of energy, because it is variable and unpredictable, and therefore provides an intermittent supply of electricity

This report attempts to set down some background, in order to assist in the assessment of the merits of wind power. It concentrates on non-domestic onshore installations, since offshore windfarms are already well developed and less contentious.

UK Wind Regime

Wind turbines start to produce power at wind speeds of 4 – 5 m/s (9-11 mph). At wind speeds higher than 25 m/s (56 mph), turbines are normally shut down for safety reasons. The optimum location for a turbine is therefore one that has wind speeds between 5 and 25 m/s for the maximum amount of time.

In a study of UK Met Office records, it was found that at any one location wind speeds below 4 m/s were experienced for 15-20% of the time; but that for more than 98% of the time half of all the locations measured had wind speeds above this critical 4 m/s. On average there was only one hour per year when 90% of the locations had wind speeds below 4 m/s at the same time.

At the other end of the scale, high wind speeds (over 25 m/s) were found to be far more unusual. The single windiest location in the UK experienced such conditions for only 2% of the time.  

 

Load Factor and Efficiency

It is useful to clarify the difference between load factor (or capacity factor) and efficiency, as the two are sometimes confused. Load factors and efficiencies are generally unrelated.

Load factor is the percentage of actual yearly output from a power station to the output if it ran at full capacity continuously over a whole year.

It might be thought that the load factor of a conventional power station would be close to 100%. However, for the years 2006-2009, the load factors of different types of power stations were:-

  • Combined cycle gas                 62%
  • Nuclear                                     61%
  • Coal fired                                  47%
  • Offshore wind                          28%
  • Onshore wind                          27%

As long as the load factor is taken into account in the design and planning of an electricity supply network, it is possible to compensate for these lower load factors.

Efficiency is very different. It is a measure of how well the plant in question converts the source of energy into electricity. It helps answer the question of, for instance, how much coal or gas we need to burn to generate a certain amount of electricity. Because the energy source for a wind turbine is free, efficiency is of far less importance for wind energy, although it will influence the number of turbines required in a particular location. However, the same principles can be applied to calculate the efficiency of a wind turbine as are used for a fossil fuel or nuclear sourced power plant.

Examples of efficiencies for various energy sources are shown below.

  • Combined cycle gas turbine stations                        48%
  • Nuclear stations                                                          38%
  • Coal fired stations                                                       36%
  • Wind turbines                                                              33%

 These figures do not include the energy used in the construction, maintenance and decommissioning of the plant itself; nor the production of the fuel, which is of course free for wind turbines. When these factors are taken into account wind turbines become relatively more attractive (see the Carbon Footprint section, below).

UK Wind Energy Production

The contribution of wind energy to electricity generation in the UK is increasing rapidly from a low base of 1.9% in 2009. This is likely to increase significantly as the substantial offshore wind farms are developed in the coming years.

 Advantages and Disadvantages of Wind Energy Production

Advantages

The two huge advantages of the production of electrical energy from wind are that the energy source is free and the supply relatively unlimited, though variable, on a UK-wide scale; and in operation the turbines do not produce any greenhouse gas emissions, nor any other pollutants. Further, once an onshore installation is complete, the land around and beneath it can continue to be used for agriculture.

The decommissioning of an onshore wind turbine is particularly straightforward. It can be taken down and carried away; and the concrete base can either be broken up and removed, or left.

There is little reason for the price of wind-produced energy to increase significantly after turbines have been erected, as the only significant costs thereafter are for maintenance and decommissioning. A large part of the cost of energy from fossil fuel and nuclear power is that of exploration, extraction and processing of the fuel itself. This is increasing as the resources become more difficult to find and extract. In addition it is thought likely that the prices of fossil and nuclear fuels will increase as global demand rises and supply becomes constrained.

Disadvantages

Despite the above, there are problems with the use of wind to produce energy. The two major ones are, firstly, the fact that at any one site wind is not constant, nor very predictable more than a few days ahead, and, secondly, that the turbines, particularly those onshore, are very visible.

Wind turbines give rise to other negative environmental effects but, even though they may be of a different type, these effects tend to be less serious than those for other large electricity generating plants, such as a coal-fired power stations or hydroelectric dams.

 The visual impact of the wind turbines is unavoidable especially in more densely populated areas. By their nature, turbines need to be placed on exposed and often elevated sites, which are difficult to hide. However, the visual impact of turbines is subjective. Some people like them; some don’t.

 A related problem is that of “flicker”, which refers to the flickering effect caused when rotating wind turbine blades cast intermittent shadows over neighbouring properties as they turn, through constrained openings such as windows, and cause nuisance, and possibly adverse health, effects. This “flicker” is only a problem when the sun shines and when a property is comparatively close to, and in the shadow of, the turbine. Thus, flicker only happens for a fairly short time each day.

 Natural England, whose remit is to protect England’s natural environment, issues guidelines on where wind turbines should be situated. The RSPB, which scrutinises hundreds of wind farm applications each year for their possible effect on birds, only objects to 6%. There is a possibility that bats in some places are killed by wind turbines, but there appears to be little evidence of this in the UK.

 Noise is perhaps the most complicated and controversial of the environmental effects caused by wind turbines. The British Wind Energy Association/RenewableUK website says:-

 “The areas suitable for such developments (windfarms) tend to be in quiet but exposed areas of countryside. A significant amount of effort is put into minimising any noise impact but it should be emphasised that typical noise levels are so low for a carefully considered site that they would normally be drowned out by a nearby stream or by a moderate breeze in nearby trees and hedgerows.”

 Other Constraints

The siting of a wind turbine or windfarm is subject to many constraints. It is unlikely that permission would be granted for a windfarm to be sited on any land with a national wildlife or landscape designation, such as a Site of Special Scientific Interest or an Area of Outstanding Natural Beauty. There are other less obvious constraints, including aircraft flightpaths, locations that impact on Ministry of Defence radar, and on radio and television broadcasting.

Location of Turbines

Currently (mid 2012), there appears to be a lot of media interest in wind turbines; much of it critical of their visual impact, but also of grants/subsidies that are given to large landowners to site turbines on their land. This is sometimes backed up by ill-understood data on the so-called “efficiency” (but generally meaning the load factor; see above) of turbines.

The UK planning system gives substantial rights to local communities (from individuals to councils) to express their views about proposed developments, and this means that, rightly or wrongly, projects can be delayed comparatively easily. When a turbine or windfarm is considered, it would seem to be sensible that a balance is maintained between the eventual advantage to be gained by the country from the development (which is likely to be related to the energy produced, and therefore number of turbines), and the work and time required to obtain planning consent (with the risk that the planning application might fail), in the face of objections by the smaller number of people directly affected by the installation.

The logical conclusion from this is that it may be more efficient to concentrate on constructing larger windfarms in less contentious areas.

However, if more wind energy developers were prepared to become involved in the organization and funding of benefits to communities affected by wind energy schemes, then it is likely that there would be far less local opposition to proposals.

Standby  Generation

Because the demand for electricity is variable over time, both throughout the day and seasonally, there is a requirement for back-up at peak times (standby generation), whatever energy source is used. No power generation system can ever operate for 100% of the time. The closest to this is probably a nuclear power station, which operates most effectively as a producer of a constant base load, but the UK’s ageing power stations are subject to unpredictable downtime for repairs. These power stations also need to be partly or totally shut down for routine maintenance, which explains their perhaps surprisingly low load factors shown earlier in this document (they are off for about two fifths of the time).

This standby capacity is required partly because it is acknowledged that any type of power station may trip out unexpectedly; and also to cover more predictable (in the short term) variations in supply, such as fluctuations in river flow; and of demand, due to weather, or the well known switching on of kettles at the end of a popular television programme. There is no reason why variations in windiness on a UK-wide scale cannot be backed up in a similar way, particularly as weather forecasting is becoming increasingly accurate over periods of days.

As the UK has an electricity grid, all windfarms are, in effect, linked.  Any variation in energy production is thus related to the whole system of windfarms; rather than those in any one area of the country. As described above, it is very unusual (about one day in fifty) for more than half of the country to have windspeeds too low to operate a wind turbine.

Clearly the wind does not blow all the time. Even though it tends to be windier in the winter and in the daytime, when more electricity is required, peaks in electricity demand and the windiest days generally do not coincide. So there will always be a requirement for standby generation capacity to cover for less windy days. The calculation/estimation of such capacity is far from straightforward. This is a topic that is not understood by all contributors to the debate! Hence, perhaps, the very different opinions on the levels of standby generation required to cover for less windy days.

Many studies have been carried out of the amount, and type, of standby capacity required for a variable energy source, such as wind; and also the associated effect on costs to the consumer. These studies generally conclude that, at least if wind energy contributes less than one fifth of the UK’s total electricity supply, there would not be a need for additional standby capacity to be constructed. 

However, work done in the USA and Australia, in particular, shows that variable energy sources can be included into a grid at levels over one fifth if more sophisticated systems of monitoring real time power demand and generation capacity are incorporated into the grid system. Such monitoring would reduce the amount of standby capacity required.

So on less windy days, some thermal power stations will be required at certain times to operate at low outputs, and therefore not to their optimum efficiencies. But if that is the price to be paid for reducing the burning of fossil fuels and the release of greenhouse gases into the atmosphere, then many people think it is worth it.

Carbon Footprint

As stated earlier, the major reason for using renewable sources of energy is to reduce emissions of greenhouse gases into the atmosphere. It is useful, therefore, to know what emissions per unit of energy the various forms of energy generation give rise to. This is not as straightforward as it might seem at first glance.

It would be easy to assume that CO2 emissions from wind power are zero. However to accurately calculate emissions, what is known as a life cycle analysis (lca) needs to be carried out. Such an analysis calculates the total emissions from the manufacture (including use of its raw materials), maintenance and decommissioning of the plant, as well as those from the extraction, processing and transport of fuel used by the plant. Not only is this very difficult to do accurately, but the results are dependent upon location in the world, and when the analysis is done.

For instance, an operation that uses a lot of electrical power – say producing steel - will give rise to a lower carbon footprint if carried out in France, where much of their electricity is produced by nuclear energy (lower carbon emissions), than if it is carried out in the UK, which currently produces the majority of its electrical power from fossil fuels. In a similar way, as the amount of renewable electricity increases with time, then the carbon footprint of any given operation will tend reduce.  On the other hand, as oil, gas and uranium become scarcer, forcing lower grade resources to be exploited, the carbon footprint for extracting them is likely to increase.

For this, and many other reasons, the results of life cycle analyses tend to be variable. However, an analysis of the results from four studies gives the following ranges of emissions for these technologies.

Lifetime emissions in gm CO2 eq per kWh

Even though these results vary within each mode of production, there is no doubt of the ranking of four of the energy sources considered. Nuclear is of a similar order to solar photovoltaic. Wind power is significantly lower than those of all the other four, including, almost certainly, nuclear.  

 

Some fossil fuel power stations may eventually be fitted with equipment that captures and stores carbon dioxide before it is released to the atmosphere (carbon capture and storage). But this technology is at best only 80-90% efficient at capturing carbon dioxide.  It increases the capital and running costs and requires a lot of energy to operate. Moreover it has yet to be demonstrated successfully on a commercial scale.

Costs

What is of great interest to generating and supply companies, consumers and politicians is the cost of supplying electricity from each source. This is notoriously difficult to calculate, as fossil fuel comprises a substantial part of the cost of production for most non-renewable generation systems, and its cost varies greatly over time. This is, of course, not the case with wind energy. However its cost to the consumer depends, currently at least, on the amount of subsidy that the government chooses to give.

This report is primarily about the physical aspects of wind generated power, rather than the financial and economic aspects. However below is some information regarding the costs of power generation, without subsidy, taken from a number of recent reports.

Cost per kWh in Pence 

 

 It can be seen that onshore wind prices per unit of energy are of the same order as those for gas, coal and nuclear. It is likely that gas prices will gradually rise, as will pressure to add carbon capture and storage (increasing costs further), and that onshore wind will therefore become more competitive. Offshore wind is more expensive, and prices will increase, as more wind farms are constructed in deeper water, but this increase may be mitigated by economies of scale and technological improvements. Solar photovoltaic is very expensive without subsidy.

Conclusions

It is advantageous to minimise our use of fossil fuels, both to mitigate climate change, and because fossil fuels, as well as uranium, are a finite resource for which demand is beginning to exceed supply. The United Kingdom is particularly well situated geographically to take advantage of the power of the wind; and so it would appear sensible to use this resource to generate electricity.

Onshore wind generation is significantly cheaper than offshore generation and so this option is more attractive financially. Its price per unit of energy today is similar to that of fossil fuel and nuclear produced energy but is likely to be relatively cheaper in future as carbon capture and storage drives up the cost of fossil fuel sources. In addition, the greenhouse gas emissions per unit of electricity generated by wind are today ten to a hundred fold less than electricity from gas and coal (without carbon capture and storage) and significantly less than nuclear.

It is inevitable that as the more remote sites are used for wind farms, more attention will be given to those closer to built-up areas or in other locations in which it has been difficult to persuade those living nearby to accept such schemes. However, if more wind energy developers were prepared to become involved in the organization and funding of benefits to communities affected by wind energy schemes, then it is likely that there would be far less local opposition to proposals.  This is important if Britain is to meet her targets for renewable energy generation and greenhouse gas emissions reductions.

Tags: