Right off, let me state that my expertise is not wind – my area is solar, particularly flat plate (thin film and crystalline technologies). But I do know this – global installed wind capacity dwarfs that of solar. In 2008 in the US according to the American Wind Energy Association (AWEA), around 1,300 MW of wind power was installed for a cumulative US total of more than 21,000 MW. Globally, solar has a cumulative installed capacity of 15,380 MWp – again, that's globally.
At around four US cents/kWh, wind provides a low cost green energy choice for utilities that fits the utilities model. Although wind is a variable power source (the wind must blow after all and sometimes it does not), utilities tend to see it as a less expensive part of the energy mix than solar, blending in with other sources. Storage has not really been an issue thus far (though this is increasingly coming to the fore).
But wind may have some solar competition – not necessarily in flat plate technologies (crystalline and thin films) – but with Concentrating Solar Thermal Power (CSP) – which is back, and possibly facing a strong future. ome CSP technologies, after all, have storage, and all serve the utility grid well during peak load times. CSP with thermal storage is becoming a viable choice for utilities when assessing renewable portfolios. CSP combined with storage (natural gas, steam, or molten salt) provides a less variable source of energy than wind.
Essentially, solar thermal electric technologies function like most single cycle or combined cycle power plants. Instead of natural gas or coal, these technologies use the sun as the fuel source that drives the engines or turbines in a plant. With all CSP technologies – as with any solar – the efficiency of the technology in converting the sun's energy to electricity is key to cost, and thus to the price of the electricity that is sold.
Development of CSP
Development of CSP, which in general converts the solar resource into heat for use by a turbine generator or a heat engine, began in the 1980s. Unfortunately, at that time it lacked the necessary incentives to drive technology improvements and cost reductions, and the early installations were not able to prove cost effectiveness, so enthusiasm waned. CSP technology development languished (though it continued to have strong supporters) until strong demand for large scale (more than one MWp) installations, utility interest, along with a silicon shortage, revived interest.
Several CSP technologies, including Solar Chimney and SNAP, with parabolic trough, are currently in commercial use. Parabolic trough technology has an expected operating life of 30 years. Plants are often natural gas hybrids that use gas fired boilers to supplement the solar energy. The disadvantages include a high initial cost, unresolved heat storage issues, and the use of wet cooling tower cooling. The technology uses about four acres per megawatt. The use of water for cooling is currently providing some controversy. Some manufacturers have answered the cost issue by using less expensive materials, and building the troughs lower to the ground.
Power tower technology generates electricity by focusing the solar resource on a tower-mounted heat exchanger, or receiver. This system uses hundreds to thousands of heliostat mirrors (sun tracking mirrors) to reflect the incident sunlight onto the receiver. With an excellent solar resource and storage, a power tower has a potential capacity factor of 65%; however, without storage the capacity factor is limited to 25%. Power tower technology operates at extremely high heats and when it fails, the failure is catastrophic. This technology uses about 8 acres per megawatt.
Dish Engine technology converts the thermal energy from the solar resource to mechanical energy and then to electrical energy in a similar fashion to conventional power plants. Dishes can reach 1,000 degrees Centigrade, have high optical efficiency and low start up losses. The systems have a modular design, making the technology appropriate for remote and hybrid installations. The technology uses about five acres per megawatt.
The early generations of the parabolic trough plants experienced problems with different system components. Three different types of collectors were used in the nine power plants installed in the 1980s and 1990s, allowing for full testing of each collector's attributes and limitations. Heat collection elements, structural elements, mirror installations, and other components all experienced some degree of failure. The systems also were not efficient in maintaining the constant solar thermal temperature necessary to generate the right amount of steam. This resulted in the use of more fossil fuels to maintain the correct amount of heat for energy conversion in the system. Generational improvements in those components have since been made that cut down on the failure rates of the overall system.
Truthfully, without the silicon shortage and the resulting constraints on supply, CSP might still be struggling – and note, the CSP industry is still in a nascent stage. Higher prices for crystalline flatplate technologies because of the high price of silicon feedstock, along with scarcity, drove the price of PV technology up in the short term. This, along with thin film's lack of capacity opened a window through which CSP – with its storage capabilities – climbed through.
Direct normal insolation
Solar thermal electric technologies require high direct normal insolation (DNI) to function. Insolation is the total amount of solar radiation that strikes a particular location over a given period of time, typically a single day. Areas such as the southwestern United States, parts of Africa, India, the Middle East, and much of Mediterranean Europe have high DNI and are good candidates for using this technology. This technology is further limited by a large area requirement, which limits its application suitability. Solar thermal systems under utility scale are potentially costly to build, maintain, and operate. Economies of scale can be achieved, but through larger systems. Advantages include:
- No emissions, except when combined with natural gas in hybrid configurations;
- Potential of producing electricity 24 hours a day (with storage);
- Large scale deployment;
- Use of typical central power plant technologies such as steam and turbine generators;
- Potentially lower costs, as theoretically, fuel costs would potentially be lower since a renewable fuel source is being used instead of costly fossil fuels;
- Hybrid plants are power plants that can use both a renewable fuel and a fossil fuel (such as natural gas) to generate electricity. On a sunny day, the solar thermal electric plant would use solar thermal energy to create electricity, while using fossil fuels at night or on cloudy days;
- Thermal storage: an option for parabolic trough and power tower solar thermal technologies.
However, as with any technology, there are disadvantages:
- Significantly capital intensive (that is, extremely expensive);
- Typically must be sited in remote areas, requiring transmission and distribution systems to be built;
- Large land requirements;
- Current high kWh cost;
- Technologies tend to have high material requirements;
- Cooling method, principally water.
As a good environmentalist, I would like to see a world powered by renewables, in which all technologies were a viable part of the mix. As an analyst and business person, however, I know that each industry wants to win – and thinks it should. In the case of wind and CSP, each has pros and cons. Wind has the significant pro of being cheaper, while CSP has the significant pro of storage capability. In these tough global economic times, winning or losing may not be the point. All technologies need to aggressively work at lowering costs so that cheaper electricity can be delivered. And, we all need subsidies to continue progressing along these lines.
In the end, wind and CSP will have to coexist, perhaps in the same large installation. And, along the way a bigger problem globally, needs to be addressed and that is transmission. Particularly in the US, where the transmission system is strained, this must be addressed for there to be continued competition, not to mention coexistence.
|About the author
Paula Mints is the principal analyst for Navigant's PV Service Market Research Program, executive editor of the Solar Outlook Newsletter, and associate director of the Energy Division.
The PV Services Department at Navigant Consulting was founded in 1974 at Strategies Unlimited, and Ms Mints moved it to Navigant in 2005. The practice is based on classic market research principles, that is, all data are primary, not secondary, and the analysis is independent and not based on the work of others.