Biofuels, Solar and Wind as Renewable Energy Systems_Benefits and Risks Episode 1 Part 7 pdf

134 A.R.B. Ferguson

The power density that is likely to be achieved when coal is used to produce

electricity has been estimated at 315 kW(e)/ha.2 Note that the power density is

there given in terms of the electrical output. Since the efficiency of producing elec￾tricity from coal is about 30%, it can be deduced that, in terms of the coal that

produces the electricity, its power density is about 315/0.30 = 1050 kW/ha. The

normal route is of course first to calculate the power density of coal itself, but that is

incidental.3

After establishing the output of electricity from wind turbines, as will be done

later, it will be appropriate to discuss whether emphasis should be placed on the

power density in terms of just the electrical power produced from the wind turbines

or whether, as is often done, that output should be uprated to take account of the

fossil fuel required to produce it. For the present, note only that as the output of

wind turbines is electricity, the first step will be to measure the power density in

terms of the electrical output, i.e. power density measured as kilowatts of electricity,

kW(e), rather than kW of fossil fuel equivalent.

Before proceeding further into the study of wind power, it will be relevant to

look briefly also at the power density of liquid fuels produced from biomass. There

are various categories of power density which can be assessed, all of them useful in

their own way. The one that is least controversial is to measure the output per hectare

of, for example, ethanol, subtracting from it only the amount of energy input that

needs to be in liquid form, e.g. as gasoline, diesel or ethanol. That gives the ‘useful’

ethanol per hectare. In such an assessment, the power density of ethanol from corn

(maize) is about 1.9 kW/ha (OPTJ 3/1).4 Incidentally ethanol from sugarcane, when

assessed on this same basis, typically achieves a power density of 2.9 kW/ha, but

soil erosion problems are worse with sugarcane than with corn, and the land that

is suitable for growing sugarcane is more restricted. Considered against the power

density of oil, which is considerably higher than the 1050 kW/ha mentioned for coal,

it is clear that these ethanol power densities are very small indeed. For example, in

the same paper, OPTJ 3/1, it is calculated that if all the U.S. corn crop were to be

used to produce ethanol, it could serve to replace only 6% of the fuel used in the

USA for transport.5

Another type of power density that can be assessed is by adding to the ethanol

output the calorific value of the by-products (e.g. dry distillers’ grains that can be

fed to cattle), and from that subtracting not only the liquid input but also the non￾liquid inputs, e.g. the heat needed for distillation (which constitute about 85% of the

inputs). The resultant ‘net energy capture’ would be a revealing figure if its value

could be agreed, but there are huge areas of uncertainty, particularly because we

need to know (a) how much of the by-product is actually going to find a use and

should therefore be counted as an output; (b) how much of the total crop can be

utilized without causing loss of soil quality. For example, in the case of corn total

yield is about 15,000 kg/ha (dry), with about half of this being grain and the other

half being stover (Pimentel and Pimentel 1996, p. 36). Growing corn is prone to

cause soil erosion. All the stover should be either left on or returned to the ground

to diminish erosion and return nutrients. Sugarcane is worse than corn at causing

soil erosion (Pimentel 1993), so a very significant proportion of the bagasse should

6 Wind Power: Benefits and Limitations 135

be returned to the soil rather than using most of it to produce the heat needed for

ethanol distillation (as tends to be done in practice).

All energy balance calculations are crude at best due to such factors, and the

‘energy balance’ of producing ethanol from corn can be assessed as either positive

or negative depending on matters of fine judgement. However, let us be clear about

what an approximate zero energy balance means. It means that producing ethanol

from biomass is not an energy transformation that produces useful energy; it is

merely a way of using other forms of available energy to produce energy in a liquid

form. The conclusion is twofold: that power density figures need to be hedged about

with precise understanding of what is being assessed, and that producing significant

quantities of liquid fuels from renewable sources is a difficult problem.

6.2 The Power Density of Electricity from Wind Turbines

In an ideal situation, where the wind always blows from the same direction, and

where docile citizens do not mind where the wind turbines are placed, the turbines

could be placed fairly close together. But in practice there are few sites where engi￾neers believe that the wind can be trusted to always come from the same direction.

Moreover there are often practical restrictions about where the wind turbines can be

placed. Due to these factors, the actual placing of wind turbines is such that about

25 ha needs to be ‘protected’ from interference by other wind turbines for each

megawatt (MW) of wind turbine capacity (Hayden 2004, pp. 145–149). Note first

that this 25 ha/MW is independent of the rated capacity of the wind turbine (e.g. two

turbines of 1 MW capacity would require 50 ha and so would one 2 MW turbine),

and secondly that the 25 ha/MW refers to the rated capacity of the wind turbines

not their actual output.

The actual output of a wind turbine, or group of wind turbines, is determined by

the capacity factor (also called load factor) that they achieve. In northern Europe

(Sweden, Denmark, Germany, the Netherlands) the mean capacity factor achieved

over two years was 22% (OPTJ 3/1, p. 4), in the UK for the years 2000–2004

capacity factors achieved were 28%, 26%, 30%, 24%, 27% for an average of 27%,6

and for the USA for the years 2000–2004 capacity factors were respectively 27%,

20%, 27%, 21%, 27% for an average of 24%.7 Nevertheless taller wind turbines

may produce some improvement, so let us use 30% as a benchmark for the USA.

This means that the protected area is 25/0.30 = 83 ha per MW of output, which

gives a power density of 1000 [kW(e)]/83 = 12 kW(e)/ha. That power density gives

an easy way to calculate how much land area would be needed to provide a certain

amount of electrical output; e.g., to produce the mean power output of a 1000 MW

power station, which delivers over the year say a mean 800 MW, the area needed

would be 800,000 [kW]/12 = 66,700 ha, or 667 km2

, or 26 km by 26 km (16 miles

by 16 miles). That is a substantial area, the ramifications of which will be considered

later, after some other measures of power density have been considered.

Also of considerable relevance is the amount of land that the wind turbines are

actually taking up, that is the land taken up by the concrete bases of the turbines and