Concentrating solar power: Energy from deserts

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Concentrating solar power (CSP), using focused solar heat to generate electricity, has moved up the agenda as a promising new energy option. In fact it’s not a new idea. The USA was one of the early pioneers with Solar 1 and Solar 2 large mirror arrays focused on so-called central “power towers” installed in the Mojave desert in the late 1970s and early 1980s. Similar projects were tested elsewhere, for example in Italy – with “heliostat” mirrors tracking the sun. But the technology was expensive. Now however, with the technology improving, energy prices rising and concerns about climate change growing, this approach is being looked at again around the world, and some major projects are under way.

The USA is one of the leaders. For example, the new 64MW Nevada Solar One solar trough system in the Nevada desert, with 760 parabolic cylinder concentrators, is expected to produce 134 GWh p.a. Pushing things on, the US Department of Energy is investing USD5.2 million in twelve “low-cost CSP” projects, and aiming to get to under 10 US cents per kWh by 2015. The market is already building up. Pacific Gas and Electric has agreed to buy up to 900MW from five new CSP plants to be built by Bright Source Energy over the next ten years in the Mojave desert at a cost of around USD3 billion. The first 100MW phase is due to be completed by 2011 – although that depends on the continuation of the federal solar investment tax subsidy, which would cover 30% of the cost. The prospects are very significant. The American Solar Energy Society says that in theory CSP plants in the south-western states of the USA “could provide nearly 7 000 GW of capacity, or about seven times the current total US electric capacity”.

Meanwhile Spain is making rapid progress, with an 11MW concentrating solar power plant project installed near Seville, built by Solucar. The solar tower is 115 metres tall and surrounded by 600 steel heliostat reflector plates which track the sun and direct its rays to a heat exchanger (receiver) at the top of the tower. This produces steam, which is used to drive turbines that can, it’s claimed, produce enough power for up to 6000 homes. Further CSP projects are planned in the area with a total of 300MW expected.

Spanish engineering group Sener Grupo De Ingenieria SA and Abu Dhabi’s pioneering alternative energy Masdar programme have announced a joint venture – Torresol Energy – which will design, build and operate CSP plants. Initially Torresol will start work on three solar power plants in Spain with an approximate combined value of EUR500 million, one of which will be a CSP central tower receiver system. The programme is seen as facilitating an anticipated 500MW of CSP projects across the “sunbelt countries” by 2012 – 1GW longer term. Sener is currently designing and building three 50MW CSP plants, with molten salt heat storage, in Spain. Having a heat store allows the power plant to continue to use solar energy overnight. Otherwise CSP units have to make use of backup gas firing to produce steam, although the same turbine generators can be used in what is then a hybrid solar–gas system. Alternatively, if it is available, local biomass could be used in a solar–biomass hybrid system.

CSP spreads

The UAE’s Masdar is not alone in the Middle East with interests in CSP. A 250kW solar parabolic trough power plant was tested in Iran in 1998 at Shiraz. And there are plans for a 17MW unit as part of a 467MW combined-cycle gas turbine (CCGT) project. Jordon has also been looking at CSP. So has Israel: it has installed an advanced high-temperature SOLASYS power tower, which uses steam to “reform” methane into hydrogen and carbon monoxide.

Equally significantly, some big new CSP projects are emerging in N Africa. For example, Egypt is building a large £30m CSP unit, near Cairo and Morocco is to build a 470 MW hybrid CSP/gas combined cycle plant at Aïn Béni Mathar, with 20MW from solar troughs.

Perhaps the most significant development is that some of the power from CSP units like this may be exported to Europe. For example, New Energy Algeria Ltd (NEAL), is planning to install a 3000km under-sea power link from the Algerian town of Adrar via the island of Sardinia, mainland Italy, Switzerland and then to the German city of Aachen, under a project provisionally entitled “Clean Power From The Desert”. The power for the project would initially come from a 150MW hybrid solar–gas plant at Hassi R’Mel in central M’Zab province and is due to come on stream in late 2009. It is seen as the first of a series of combined-cycle hybrid plants that NEAL says should have capacity of 500MW, or 5% of national generating capacity, by 2010. As solar technology improves, they would expand the solar proportion and then establish pure solar generation plants without the need for gas.

The economics of long-distance power transmission are not seen as prohibitive, if use is made of high voltage direct current (HVDC) links, which are said to involved energy losses of only about 1-2% per 1000 km. So we may see more projects like this – across the world. For example, according to the DESERTEC group, the whole of China could be powered by CSP plants in the desert regions in the north of the country. Most of those regions are less than 3000km from places like Beijing and Shanghai and it is feasible and cost effective to transmit solar electricity by HVDC over those distances. There are plans by German company Solar Millennium to build a 50MW USD162 million pilot CSP plant at Ordos in Mongolia to the north of China, followed by a 1GW project by 2020, costing USD2.5 billion.

CSP prospects

A study by the German Aerospace Center, Institute of Technical Thermodynamics, claimed that that “a first 10MW plant in Jordan could produce electricity at about USD0.18/kWh. In the period up to 2010, the cost of solar electricity in newly installed plants will drop to less than USD0.10/kWh, in 2020 to USD0.06/kWh and in 2030 to USD0.05/kWh.” It suggested that, given the adoption of HVDC links, CSP could have “a competitive price of about EUR0.05/kWh in Europe for import of solar electricity” and that, “starting between 2020 and 2025 with a transfer of 60TWh pa, solar electricity imports could subsequently be extended to 700 TWh pa by 2050”.
So the prospects look very good. And some new technologies that are emerging may make it even better. For example, the US Sandia Labs and Stirling Energy Systems have developed a focused solar dish system feeding a stirling engine, which has established a new solar-to-grid energy conversion efficiency of 31.25%. The previous record, which has stood since 1984, was 29.4%.

What are the drawbacks to CSP? First, you need cooling water to run the plants. However, most of the north African projects are likely to be sited within reasonable piping distance of the Mediterranean, with desalination of sea water being one of the options.

Second, long distance HVDC electricity transmission does open up the potential for terrorists attack and also might require building new grid links over land to service Europe. But HVDC isn’t the only option. The electricity produced by a desert CSP plant could be used on site to generate hydrogen by electrolysis, or high-temperature units can be used to decompose water directly into hydrogen. The hydrogen could then be stored and dispatched by pipe or tanker to where it was needed. An even more intriguing idea is to convert it, by reaction with carbon dioxide from the air, to methanol or some other easily transportable liquid fuels.

Third, there may be political objections to CSP – for example the EU would be still be reliant on overseas energy sources. But with oil prices rising, replacing oil imports with imported solar should be a winning strategy and, as long as the commercial relationships are not exploitative, it could benefit all involved.
For more on CSP see DESERTEC and Europa.

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