Queries On Solar Power Generated Stirling Engine

I'm confused! In Chapter 6, you said that the best photovoltaic panels deliver 20 W/m2 on average, in a place with British sunniness. Presumably in the desert the same panels would deliver 40 W/m2. So how come the concentrating solar power stations deliver only 15-20 W/m2? Surely concentrating power should be even better than plain flat panels?

Good question. The short answer is no. Concentrating solar power does not achieve a better power per unit land area than flat panels. The concentrating contraption has to track the sun, otherwise the sunlight won't be focused right; once you start packing land with sun-tracking contraptions, you have to leave gaps between them; lots of sunlight falls through the gaps and is lost. The reason that people nevertheless make concentrating solar power systems is that, today, flat photovoltaic panels are very expensive, and concentrating systems are cheaper. The concentrating people's goal is not to make systems with big power per unit land area. Land area is cheap (they assume). The goal is to deliver big power per dollar.

But if flat panels have bigger power density, why don't you describe covering the Sahara desert with them?

Because I am trying to discuss practical options for large-scale sustainable power production for Europe and North Africa by 2050. My guess is that by 2050, mirrors will still be cheaper than photovoltaic panels, so concentrating solar power is the technology on which we should focus.

What about solar chimneys?

A solar chimney or solar updraft tower uses solar power in a very simple way. A huge chimney is built at the centre of an area covered by a transparent roof made of glass or plastic; because hot air rises, hot air created

Figure 25.9. A 25 kW (peak) concentrator photovoltaic collector produced by Californian company Amonix. Its 225 m2 aperture contains 5760 Fresnel lenses with optical concentration x 260, each of which illuminates a 25%-efficient silicon cell. One such collector, in an appropriate desert location, generates 138 kWh per day - enough to cover the energy consumption of half an American. Note the human providing a scale. Photo by David Faiman.

Figure 25.9. A 25 kW (peak) concentrator photovoltaic collector produced by Californian company Amonix. Its 225 m2 aperture contains 5760 Fresnel lenses with optical concentration x 260, each of which illuminates a 25%-efficient silicon cell. One such collector, in an appropriate desert location, generates 138 kWh per day - enough to cover the energy consumption of half an American. Note the human providing a scale. Photo by David Faiman.

in this greenhouse-like heat-collector whooshes up the chimney, drawing in cooler air from the perimeter of the heat-collector. Power is extracted from the air-flow by turbines at the base of the chimney. Solar chimneys are fairly simple to build, but they don't deliver a very impressive power per unit area. A pilot plant in Manzanares, Spain operated for seven years between 1982 and 1989. The chimney had a height of 195 m and a diameter of 10 m; the collector had a diameter of 240 m, and its roof had 6000 m2 of glass and 40 000 m2 of transparent plastic. It generated 44 MWh per year, which corresponds to a power per unit area of 0.1 W/m2. Theoretically, the bigger the collector and the taller the chimney, the bigger the power density of a solar chimney becomes. The engineers behind Manzanares reckon that, at a site with a solar radiation of 2300kWh/m2 per year (262 W/m2), a 1000m-high tower surrounded by a 7km-diameter collector could generate 680 GWh per year, an average power of 78 MW. That's a power per unit area of about 1.6 W/m2, which is similar to the power per unit area of windfarms in Britain, and one tenth of the power per unit area I said concentrating solar power stations would deliver. It's claimed that solar chimneys could generate electricity at a price similar to that of conventional power stations. I suggest that countries that have enough land and sunshine to spare should host a big bake-off contest between solar chimneys and concentrating solar power, to be funded by oil-producing and oil-consuming countries.

Figure 25.10. The Manzanares prototype solar chimney. Photos from solarmillennium.de.

What about getting power from Iceland, where geothermal power and hy-droelectricity are so plentiful?

Indeed, Iceland already effectively exports energy by powering industries that make energy-intensive products. Iceland produces nearly one ton of aluminium per citizen per year, for example! So from Iceland's point of view, there are great profits to be made. But can Iceland save Europe? I would be surprised if Iceland's power production could be scaled up enough to make sizeable electricity exports even to Britain alone. As a benchmark, let's compare with the England-France Interconnector, which can deliver up to 2 GW across the English Channel. That maximum power is equivalent to 0.8 kWh per day per person in the UK, roughly 5% of British average electricity consumption. Iceland's average geothermal electricity generation is just 0.3 GW, which is less than 1% of Britain's average electricity consumption. Iceland's average electricity production is 1.1 GW. So to create a link sending power equal to the capacity of the French inter-connector, Iceland would have to triple its electricity production. To provide us with 4 kWh per day per person (roughly what Britain gets from its own nuclear power stations), Iceland's electricity production would have to increase ten-fold. It is probably a good idea to build interconnectors to Iceland, but don't expect them to deliver more than a small contribution.

Figure 25.11. More geothermal power in Iceland. Photo by Rosie Ward.

Notes and further reading page no.

178 Concentrating solar power in deserts delivers an average power per unit area of roughly 15 W/m2. My sources for this number are two companies making concentrating solar power for deserts.

www. stirlingenergy. com says one of its dishes with a 25 kW Stirling engine at its focus can generate 60 000 kWh/y in a favourable desert location. They could be packed at a concentration of one dish per 500 m2. That's an average power of 14 W/m2. They say that solar dish Stirling makes the best use of land area, in terms of energy delivered.

www.ausra.com uses flat mirrors to heat water to 285°C and drive a steam turbine. The heated, pressurized water can be stored in deep metal-lined caverns to allow power generation at night. Describing a "240 MW(e)" plant proposed for Australia (Mills and Lievre, 2004), the designers claim that 3.5 km2 of mirrors would deliver 1.2 TWh(e); that's 38 W/m2 of mirror. To find the power per unit land area, we need to allow for the gaps between the mirrors. Ausra say they need a 153 km by 153 km square in the desert to supply all US electric power (Mills and Morgan, 2008). Total US electricity is 3600TWh/y, so they are claiming a power per unit land area of 18 W/m2. This technology goes by the name compact linear fresnel reflector (Mills and Morrison, 2000; Mills et al., 2004; Mills and Morgan, 2008). Incidentally, rather than "concentrating solar power," the company Ausra prefers to use the term solar thermal electricity (STE); they emphasize the benefits of thermal storage, in contrast to concentrating photovoltaics, which don't come with a natural storage option.

Trieb and Knies (2004), who are strong proponents of concentrating solar power, project that the alternative concentrating solar power technologies would have powers per unit land area in the following ranges: parabolic troughs, 14-19 W/m2; linear fresnel collector, 19-28 W/ m2; tower with he-liostats, 9-14 W/m2; stirling dish, 9-14 W/m2.

There are three European demonstration plants for concentrating solar power. Andasol - using parabolic troughs; Soliucar PS10, a tower near Seville; and Solartres, a tower using molten salt for heat storage. The Andasol parabolic-trough system shown in figure 25.4 is predicted to deliver 10 W/m2. Solicar's "11MW" solar tower has 624 mirrors, each 121 m2. The mirrors concentrate sunlight to a radiation density of up to 650 kW/ m2. The receiver receives a peak power of 55 MW. The power station can store 20MWh of thermal energy, allowing it to keep going during 50 minutes of cloudiness. It was expected to generate 24.2 GWh of electricity per year, and it occupies 55 hectares. That's an average power per unit land area of 5 W/m2. (Source: Abengoa Annual Report 2003.) Solartres will occupy 142 hectares and is expected to produce 96.4GWh per year; that's a power density of 8 W/m2. Andasol and Solartres will both use some natural gas in normal operation.

179 HVDC is already used to transmit electricity over 1000-km distances in South Africa, China, America, Canada, Brazil, and Congo. Sources: Asplund (2004), Bahrman and Johnson (2007). Further reading on HVDC: Carlsson (2002).

Figure 25.12. Two engineers assembling an eSolar concentrating power station using heliostats (mirrors that rotate and tip to follow the sun). esolar. com make medium-scale power stations: a 33 MW (peak) power unit on a 64 hectare site. That's 51W/m2 peak, so I'd guess that in a typical desert location they would deliver about one quarter of that: 13 W/ m2.

Figure 25.12. Two engineers assembling an eSolar concentrating power station using heliostats (mirrors that rotate and tip to follow the sun). esolar. com make medium-scale power stations: a 33 MW (peak) power unit on a 64 hectare site. That's 51W/m2 peak, so I'd guess that in a typical desert location they would deliver about one quarter of that: 13 W/ m2.

Figure 25.13. A high-voltage DC power system in China. Photo: ABB.

179 Losses on a 3500km-long HVDC line, including conversion from AC to DC and back, would be about 15%. Sources: Trieb and Knies (2004); van Voorthuy-sen (2008).

182 According to Amonix, concentrating photovoltaics would have an average power per unit land area of 18 W/m2. The assumptions of www.amonix. com are: the lens transmits 85% of the light; 32% cell efficiency; 25% collector efficiency; and 10% further loss due to shading. Aperture/land ratio of 1/3. Normal direct irradiance: 2222 kWh/m2/year. They expect each kW of peak capacity to deliver 2000kWh/y (an average of 0.23 kW). A plant of 1 GW peak capacity would occupy 12 km2 of land and deliver 2000 GWh per year. That's 18 W/m2.

- Solar chimneys. Sources: Schlaich J (2001); Schlaich et al. (2005); Dennis (2006), www.enviromission.com.au,www.solarairpower.com.

183 Iceland's average geothermal electricity generation is just 0.3 GW. Iceland's average electricity production is 1.1GW. These are the statistics for 2006: 7.3 TWh of hydroelectricity and 2.6 TWh of geothermal electricity, with capacities of 1.16 GW and 0.42 GW, respectively. Source: Orkustofnun National Energy Authority [www. os. is/page/energystatistics].

Further reading: European Commission (2007), German Aerospace Center (DLR) Institute of Technical Thermodynamics Section Systems Analysis and Technology Assessment (2006), www.solarmillennium.de.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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