High-Temperature Solar Thermal (HTST) Technology: An Overview and Evaluation

This report looks at high-temperature solar thermal (HTST) technology, with the four main designs being considered: parabolic dish, parabolic trough, power tower, and linear Fresnel. First, a description of HTST technology is provided, and the commercialisation of HTST technology is examined. HTST technology is then evaluated from social, environmental and economic perspectives, with consideration given to both positive and negative issues. The economic evaluation includes a comparison with coal power technology. Finally, the limitations of HTST technology are outlined, and barriers to implementation in Australia are discussed. From the information presented, it can be seen that HTST has many benefits, and vast potential, especially in Australia. However, the high cost and a lack of government support continues to prevent its rapid uptake.

High-Temperature Solar Thermal (HTST) Technology Overview

Solar thermal technologies are categorized as low-temperature, medium-temperature, or high-temperature. High-temperature solar thermal (HTST), also known as concentrating solar thermal (CST), is used for electrical power generation. HTST power plants are a lot like traditional fossil fuel power plants, but the important difference is that they obtain their energy input from the sun, instead of from fossil fuels. HTST systems have two main components: the collector / concentrator, and the receiver / absorber. The collector is a mirror with the function of collecting solar energy and concentrating this energy (by reflection) toward a centralised receiver, which contains a working fluid that absorbs the concentrated solar energy. The four main HTST designs are: parabolic trough, parabolic dish, power tower, and linear Fresnel.

Parabolic Trough
The parabolic trough (Figure 1) is the most common type of HTST system(1). The mathematical properties of a parabola curve are such that it reflects all incident sunlight onto a focal point, and thus the concentration of solar energy can be maximised. However, parabolic troughs do not fully exploit these properties, being parabolic only in one dimension. The concentration ratio is typically 8 to 80 times the incident intensity, with operating temperatures ranging from 260 to 400 degrees C, and a maximum conversion (Carnot) efficiency of 56%(2). The working fluid contained in the tubular receiver is usually synthetic oil (molten salt or water/steam may also be used). Heat absorbed by this oil is used to generate steam via heat exchangers (Rankine cycle), in order to power a steam turbine, which drives an electrical generator. A power plant utilising this design will be comprised of many parallel rows of troughs, usually orientated on a north-south axis and each with a single-axis sun tracking system.

Parabolic Dish
The parabolic dish (Figure 2) is based on the same principle as the parabolic trough, however the dish is parabolic in two dimensions, and the incident sunlight is concentrated toward a single focal point. As a result, the typical concentration ratio is much higher at 800 to 8000, with operating temperatures ranging from 500 to 1200 degrees C, and a maximum conversion efficiency of 80%(2). The working fluid is a gas, such as hydrogen or air, which is used to generate electricity via a small Stirling or Brayton engine attached to the receiver. Parabolic dishes employ double-axis sun tracking systems.

Power Tower
The power tower (Figure 3) arrangement consists of a large array of double-axis sun-tracking collectors on the ground (heliostats), which reflect and concentrate the incident solar energy onto a central tower-mounted receiver. The concentration ratio is typically 600 to 1000, with operating temperatures ranging from 500 to 800 degrees C, and a maximum conversion efficiency of 73%(2). The working fluid is usually molten salt, synthetic oil, or liquid sodium (earlier systems used water/steam). As with the parabolic trough, power generation is achieved via Rankine cycles and a steam turbine.

Linear Fresnel
The linear Fresnel (Figure 4) arrangement utilises a series of long (slightly curved or flat) collectors and one or more linear receivers. The design might be viewed as incorporating aspects of the parabolic trough and the power tower. Like the parabolic trough, the receiver runs the length of the collectors, however it is physically independent of them and fixed in position (as with the power tower), which leads to a simpler design. Single-axis tracking is used. The concentration ratio, operating temperature range, and maximum efficiency are similar to the parabolic trough(2).

Energy Storage & Base Load Power
One advantage of HTST over other renewable energy technologies, such as solar photovoltaics and wind turbines, is that the captured energy can be stored more easily. Because of this, intermittency is less of an issue, and HSTS can provide base load power(4). Molten salt is a proven storage medium(5)(6), with the obvious advantage that it can simultaneously be used as the working fluid. Other options include the use of concrete or phase change materials(5), and the dissociation of ammonia to produce hydrogen(7). By utilising energy storage, HTST can replace traditional fossil fuel power plants. On the other hand, if energy storage is not used, HTST power plants can provide base load power by supplementing with fossil fuels (usually natural gas).

Commercial Status of HTST
The United States and Spain are the world leaders in HTST for power generation. Each currently has 6 operational plants (see Table 1), with a total capacity of 430 MW and 182 MW respectively(8).

Operational HTST Power Plants in the USA and Spain (8)

Spain has 20 plants under construction, which account for 1617 MW of the 1757 MW (92%) of capacity under construction worldwide(8). All of the plants under construction in Spain are expected to come online by the end of 2010, and all will use parabolic trough technology, except for one 17 MW plant, which will use the power tower arrangement. Spain also has another 19 plants announced, totalling 1080 MW. Of these, 15 will be parabolic trough, with 5 of those 15 using energy storage(8).

The USA has 29 plants announced, totalling 8546 MW(8). Of these, 15 will be parabolic trough, 8 will be power tower, 2 will be parabolic dish, and 2 will be linear Fresnel. So although parabolic troughs remain the favourite HTST technology, power towers are becoming increasingly attractive, as are parabolic dishes, at least in the USA.

In Australia, large-scale HTST power installations are yet to be seen, however this is likely to change in the near future, as Australia is actively involved in R&D and commercialisation. Australia’s first HTST power station was constructed by the Australian National University, at White Cliffs, NSW, in 1981. It was comprised of 14 parabolic dishes, with a total capacity of just 25 kW, before Solar Systems Pty. Ltd. converted the dishes to PV in 1997(9)(10). Solar Systems Pty. Ltd. has also recently constructed parabolic dish power stations at Hermannsburg (192 kW), Yuendumu (240 kW), Lajamanu (288 kW), and Umawa (220 kW), and although the dishes use PV technology, they are also capable of high temperature operation, and the CSIRO has been using the technology for this purpose(11).

The CSIRO’s National Solar Energy Centre (NSEC), launched in 2006, is the only multi-collector installation of its type in Australia(12). It consists of a power tower with a capacity of 0.5 MW, and a linear Fresnel system.

In 2003, Solar Heat & Power Pty. Ltd. begun the construction of a 40 MW compact linear Fresnel plant, located adjacent to the 2000 MW coal-fired power station in Liddell(13). The company is now designing a stand-alone 240 MW system.

The Australian National University’s ‘Big Dish’ (Figure 5) is the world’s largest parabolic dish technology, with an aperture of 400m2(14). The ANU and Wizard Power Pty. Ltd. have entered into a partnership to commercialise the Big Dish, with plans to construct a demonstration plant consisting of up to 20 dishes, each with an electrical generation capacity of 100 kW.

Environmental Evaluation

HTST is a renewable energy technology, however this does not mean that it is necessarily an environmentally sustainable technology in all instances. The environmental impact of HTST depends on how it is implemented. Here we look at several environmental issues associated with HTST technology.

Land use: HTST requires large areas of cleared flat land. The amount of land required of course varies depending on the size of the plant, the chosen HTST design, and whether energy storage is employed. A typical parabolic trough plant requires about 5 to 10 acres of land per MW of capacity(16), and the land required for a typical parabolic dish plant is at the lower end of this range(17). For comparison, a typical coal-fired plant requires about 1 acre per MW of capacity(18), but this does not include the land area required for mining coal.

Conservation of biodiversity: due to the land and solar energy requirements of HTST, the Mojave Desert in California has become a potential hotspot for HTST companies and developers, in what has been described as a “…California gold rush-like scenario unfolding in the desert”(19). However, there are mounting concerns about the potential impact on rare and protected species in the area, such as the desert tortoise, the Mojave fringe-toed lizard and Nelson’s bighorn sheep(20). These concerns have already resulted in some HTST project plans being abandoned(21), but the mandate for emissions reductions remains a strong driving force behind HTST development, and decision-makers have indicated that this mandate will take precedence over biodiversity conservation(19).

Water use: power plants that utilise Rankine steam cycles need to be cooled, and HTST power plants are no exception. The amount of water used for HTST power generation depends on the cooling method and chosen HTST design. A wet-cooled power tower plant uses about 2250 Lt/MWh (about the same as a typical wet-cooled coal or nuclear plant), and a parabolic trough plant uses about 3650 Lt/MWh(22). Dry-cooling (air-cooling) is an option that eliminates 90% of water use, but it also decreases energy output and increases the cost of producing electricity by an amount that depends on the location and chosen HTST design(22). Parabolic dishes are inherently dry-cooled, and thus use virtually no water(23).

Air pollution: it has been calculated that HTST results in lifecycle greenhouse gas emissions of 10g to 80g CO2e/kWh, depending on the chosen HTST design(24). However, these emissions must be offset against the ‘avoided emissions’ i.e. emissions that would otherwise have come from the combustion of fossil fuels or nuclear fission. It has been calculated that lifecycle greenhouse gas emissions (including fuel production) for coal, gas (CCGT) and nuclear power are 974g CO2e/kWh, 464g CO2e/kWh, and 15g CO2e/kWh respectively(25). It should be noted that the calculated lifecycle greenhouse gas emissions for nuclear power do not include emissions associated with the permanent storage of high-level radioactive waste, and so the actual figure would necessarily be higher than that cited above. Based on the above figures, it can be seen that replacing coal power with HTST results in emissions reductions of about 95% on average, and replacing gas power with HTST results in emissions reductions of about 90% on average. HTST also results in the elimination of SOx / NOx and other undesirable emissions during operation. Thus, in terms of greenhouse gas emissions, and air pollution in general, HTST is a beneficial technology.

Social Evaluation

Aesthetic impacts: due to the sheer size of HTST installations, some people might consider them to be visually offensive. This of course depends on where installations are sited, and who is looking at them. In HTST hotspots, such as the Mojave Desert, “nearby residents and national park visitors will (also) face the burden of increased traffic, pollution, noise, and infrastructure…”(26). All of these things diminish the aesthetic quality of the immediate environment, and can thus result in decreased well-being for some people.

Remote Area Power Supply (RAPS): currently, many remote communities do not have access to a reliable power supply, and this is a social inequality. Some remote communities use stand-alone photovoltaic systems (or small wind turbines) with a diesel generator for backup, and a battery bank for energy storage. HTST has so far proven best suited to large-scale applications, and thus has not been deployed for RAPS. As a result, the benefits of HTST are not yet realised by those who most in need of a reliable power supply.

Energy Security: for HTST to be viable, a certain amount of solar irradiation must be available for harnessing at the site. As Figure 6 shows, solar resources are abundant in the USA, Mexico, northern and southern Africa, the Middle East, and Australia. These regions / countries can utilise HTST, and thus obtain a larger proportion of domestic power from an indigenous energy resource. This would necessarily increase energy security, and therefore contribute to a reduction in social and political tensions associated with energy insecurity.

Public acceptance: there are few if any studies that include specific information on the public acceptance of HTST power. However, it would not be unreasonable to assume that the level of public acceptance of HTST is somewhat similar to that of solar technologies in general. Solar technology is one of the most widely recognised types of renewable energy technology, and the most positively regarded(28). It is argued that HTST will have a high level of public acceptance, provided that environmental issues such as land use and conservation of biodiversity are addressed prior to implementation.

Economic Evaluation

The cost of HTST power depends on system design and power plant siting. As we have seen, the parabolic trough is the most utilised design, followed by the power tower and linear Fresnel, with the parabolic dish seldom being utilized so far. This is indicative of the current cost of each design relative to the others. With regard to power plant siting, energy will be cheaper to produce where solar resources are plentiful.

Currently, the cost of HTST (not parabolic dish) power is between 125 and 225 USD per MWh(29)(30), which gives an average cost of about 175 USD per MWh. The lower end of this range represents larger parabolic trough installations (under construction or recently completed) that utilise energy storage, and are sited in locations that receive high levels of solar radiation. This is compared to the cost of coal power, which can range between 30 and 70 USD per MWh (31)(32)(33), giving an average cost of about 50 USD per MWh. Thus, HTST power is about 3.5 times as expensive as coal power on average.

The cost break-downs of HTST and coal power are quite different. The majority of the cost of HTST is made up of the capital cost, and fuel costs are nil. The costs of coal power, on the other hand, are more evenly distributed between capital costs, fuel and O&M(32) (with O&M accounting for the smallest proportion), however the ratio of course depends on certain factors such as whether the fuel is indigenous or imported.

It should be noted that the figures for coal power cited above do not include the cost of carbon capture and storage (CCS) technology, carbon pricing, or negative externalities. It is estimated that CCS will increase the cost of coal power by between 37% and 91%(34). If we take the average of this range (i.e. 64%) and apply it to the average cost of coal calculated above, we find that coal power would cost about 82 USD per MWh on average, with CCS. A carbon tax of 32 USD per MWh would have the same effect. In both cases, the cost of coal power would still be half that of HTST. Negative externalities e.g. environmental pollution / degradation are difficult to quantify, but it could be argued that if they were also factored in, HTST would start to become cost-competitive with coal.

When comparing the cost of different technologies, projections should also be considered. Whilst the cost of coal power will probably increase due to carbon taxes / carbon trading legislation, the cost of HTST is expected to decrease. Significant cost reduction potential exists in three key areas: increased volume production, plant scale up, and technological advance(35). Accordingly, projections suggest that the cost of HTST power will be reduced down to 50 USD per MWh (equal to the cost of coal power now), at a total installed capacity of 40 GW, some time between 2020 and 2025(35). Energy policy can play an important role in the uptake of HTST. For example, Spain has a set target of 500 MW of installed HTST capacity by 2010, and this target is supported by a generous HTST feed-in tariff(35). This is one of the main reasons why HTST has experienced rapid growth in Spain.

HTST projects boost employment. One feasibility study found that HTST power plants require a workforce 67% larger than that required for a combined cycle plant, and that “investment in CSP power plants delivers greater return to California in both economic activity and employment than corresponding investment in natural gas equipment”(35).

Barriers to the Implementation of HTST in Australia

The average amount of solar radiation in Australia ranges from 1500 to 1900 kWh/m2/yr, the highest of any continent in the world(11). Based on this information, it has been calculated that Australia’s total primary energy consumption in 2006 (5500 PJ) could have been met by 4000 km2 of solar collectors, assuming an average conversion efficiency of 20%(11). If constructed as a power station with 20% land coverage, then just 138 km2 of land would be required, which is 0.000018% of the total land area of Australia.

This begs the question: why are there so few HTST installations in Australia, considering Australia’s solar energy resources and HTST expertise are among the best in the world?

One obvious answer to the question is that HTST is not yet cost competitive with other available energy technologies. This is particularly true in Australia, because coal is so cheap and abundant. As well, Australia’s wind resources are excellent(36) and wind power has so far proven the most economic of the ‘new renewables’ in Australia(37). These two factors have made it difficult for HTST to penetrate the market for large-scale power generation.

The other answer to the question is that government incentives are lacking. David Mills, co-founder of Ausra and Solar Heat & Power Pty. Ltd., has moved his business offshore to California, and cites the lack of government support in Australia as a key reason(38). Between 1997 and 2007, the Australian Government’s Mandatory Renewable Energy Target resulted in a wind power boom, but it did little, if anything, for HTST(37). To this day, there is still no national target for installed HTST capacity, still no HTST feed-in tariff, and still no price on carbon emissions. Only very recently has the Australian Government acknowledged the potential of HTST, through the Solar Flagships Program, which provides funding for two HTST power plants(39).

Another barrier, alluded to earlier, is that HTST is not suited to small stand-alone applications in the same way that solar PV and small wind turbines are. Thus, this potential market is not available for exploitation, at least not yet.

Conclusion

This report has described the four main HTST system designs. The parabolic trough currently dominates the market, but power tower and linear Fresnel systems are increasingly being utilised and scaled up. Parabolic dishes are the most expensive design, and thus the least used, despite the fact that they are the most efficient, and use less land and water. Parabolic dishes may become the favoured design in the future if costs can be reduced. Any HTST design can provide base load power if energy storage is employed.

HTST requires large areas of cleared flat land, and can cause significant environmental damage if inappropriately sited. Best-practice would prescribe that HTST installations be located away from fragile ecosystems, and preferably on already degraded land. Water use is another issue that must be considered, particularly in desert / dry locations. The potential environmental and aesthetic impacts of HTST are generally more than offset by the benefits, which include significant reductions in greenhouse gas emissions and other air pollutants, and increased energy security. Consequently, HTST has a high level of public acceptance.

The cost of HTST power is significantly higher than that of coal power, particularly in Australia. If the costs of CCS and negative externalities are factored in, then HTST starts to become more cost competitive with coal. The cost of HTST is projected to decrease significantly over the next 10 to 15 years due to production volume increases, plant size scale up, and technological advance. HTST projects are labor intensive and thus could be especially beneficial during times of higher unemployment, such as now.

The USA and Spain are world leaders in HTST deployment, while Australia is actively involved with R&D and commercialisation of cutting-edge HTST technology. In Australia, HTST faces strong competition with fossil fuel and wind power. So far, government incentives have been inadequate. If Australia is to take advantage of its abundant solar resources, new national and state energy policies must be introduced. Such policies would include a national target for installed HTST capacity, and a feed-in tariff for HTST power.

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