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Solar thermal energy

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Solar thermal energy is a technology for harnessing solar energy for heat. This is very different from solar photovoltaics, which convert solar energy directly into electricity. Solar thermal collectors are characterized by the US Energy Information Agency as low, medium, or high temperature collectors. Low temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production.

Low-Temperature Collectors

Of the of solar thermal collectors produced in the United States in 2006, were of the low-temperature variety. Low-temperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the fluid to transfer the heat to its destination.

Medium-Temperature Collectors

These collectors could be used to produce approximately 50% of the hot water needed for residential and commercial use in the United States. In the United States, a typical system costs $5000-$6000 and 50% of the system qualifies for a tax credit. With this incentive, the payback time for a typical household is nine years. A crew of one plumber and two assistants with minimal training can install two systems per week. The typical installation has negligible maintenance costs and reduced a household's operating costs by $6 per person per month, reducing CO2 produced for hot water heating by 1 ton/year (if replacing natural gas for hot water heating) or 3 ton/year (if replacing electric hot water heating). Medium-temperature installations can use any of several designs: common designs are pressurized glycol, drain back, and batch systems.

High-Temperature Collectors: Concentrated solar power (CSP) plants

Where temperatures below about 95°C are sufficient, as for space heating, flat-plate collectors of the nonconcentrating type are generally used. The fluid-filled pipes can reach temperatures of 150 to 220 degrees Celsius when the fluid is not circulating. This temperature is too low for efficient conversion to electricity, since the efficiency of any heat engine increases as the temperature of its heat source increases. In concentrated solar power plants, the solar radiation is concentrated by mirrors or lenses to obtain the higher temperature.

Since the CSP plant generates first heat, it is possible to store the heat before conversion to electricity. With current technology, storage of heat is much cheaper and efficient than storage of electricity. In this way, the CSP plant can produce electricity day and night. If the CSP site has predictable solar radiation, then the CSP plant becomes a reliable power plant. Reliability can further be improved by installing a back-up system that uses fossil energy. The back-up system can reuse most of the CSP plant, which decreases the cost of the back-up system.

With reliability, unused desert, no pollution and no fuel costs, the only obstacle for large deployment for CSP is cost. Although only a small percentage of the desert is necessary to meet global electricity demand, still a large area must be covered with mirrors or lenses to obtain a significant amount of energy. An important way to decrease cost is the use of a simple design.

During the day the sun has different positions. If the mirrors or lenses would not move, then the focus of the mirrors or lenses would change. Therefore it seems unavoidable that there is a tracking system that follows the position of the sun (for solar photovoltaics a solar tracker is only optional). The tracking system increases cost. With this in mind, the different designs can be distinguished in how they concentrate the light and track the position of the sun.

Parabolic trough designs

Parabolic trough power plants use a curved trough which reflects the direct solar radiation onto a receiver (also called absorber or collector) running along above the trough. The trough is parabolic in one direction and just straight in the other direction. For change of position of the sun orthogonal to the receiver, the whole trough tilts so that direct radiation remains focused on the receiver. However, a change of position of the sun parallel to the trough, does not require adjustment of the mirrors, since the light is just concentrated on another part of the receiver. So, the trough design avoids a second axis for tracking.

A substance (also called heat transfer fluid) passes through the receiver and becomes hot. Used substances are synthetic oil, molten salt and pressurized steam. The receiver can be in a vacuum chamber of glass. The light will shine through the glass and vacuum, but the vacuum will prevent loss of the collected heat. The substance with the heat is transported to a heat engine where the heat is converted to electricity.

Full-scale parabolic trough systems consist of many such troughs laid out in parallel over a large area of land.

Since 1985 a solar thermal system using this principle is in full operation in California in the United States. It is called the SEGS system. Other CSP designs lack this kind of long experience and therefore it can currently be said that the parabolic trough design is the only proven CSP technology.

The Solar Energy Generating System (SEGS) is a collection of nine plants with a total capacity of 350MW. It is currently the largest operational solar system (both thermal and non-thermal). A newer plant is Nevada Solar One plant with a capacity of 64MW. Under construction are Andasol 1 and Andasol 2 in Spain with each site having a capacity of 50MW. Note however, that those plants have heat storage which requires a smaller (but better utilized) generator. With day and night operation Andasol 1 produces more energy than Nevada Solar One.

553MW new capacity is proposed in Mojava Solar Park, California. Furthermore, 59MW hybrid plant with heat storage is proposed near Barstow, California . Near Kuraymat in Egypt, some 40MW steam is used as input for a gas powered plant. Finally, 25MW steam input for a gas power plant in Hassi R'mel, Algeria.

Power tower designs

Power towers (also known as 'central tower' power plants or 'heliostat' power plants) use an array of flat, moveable mirrors (called heliostats) to focus the sun's rays upon a collector tower (the receiver).

The advantage of this design above the parabolic trough design is the higher temperature. Higher temperatures can be converted to electricity more efficiently and can be stored cheaper for later use. Furthermore, there is less need to flatten the area. In principle a power tower can be built on a hillside. Mirrors can be flat and plumbing is concentrated in the tower. The disadvantage is that each mirror must have its own dual-axis control, while in the parabolic trough design one axis can be shared for a large array of mirrors.

A working tower power plant is PS10 in Spain with a capacity of 11MW.

The 15MW Solar Tres plant with heat storage is under construction in Spain. In South Africa, a 100MW solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m². A 10MW power plant in Cloncurry Australia (with purified graphite as heat storage located on the tower directly by the receiver). The company BrightSourceEnergy has announced to build 400MW in California with the Power Tower technology of Luz II.

Out of commission are the 10MW Solar One (later redeveloped and made into Solar Two) and the 2MW Themis plants.

A cost/performance comparison between power tower and parabolic trough concentrators was made by the NREL which estimated that by 2020 electricity could be produced from power towers for 5.47 ₡/kWh and for 6.21 ₡/kWh from parabolic troughs. The capacity factor for power towers was estimated to be 72.9% and 56.2% for parabolic troughs. There is some hope that the development of cheap, durable, mass produceable heliostat power plant components could bring this cost down.

Dish designs

A dish system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a receiver captures the heat and transforms it into a useful form. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator .

The advantage of a dish system is that it can achieve much higher temperatures due to the higher concentration of light (as in tower designs). Higher temperatures leads to better conversion to electricity and the dish system is very efficient on this point. However, there are also some disadvantages. Heat to electricity conversion requires moving parts and that results in maintenance. In general, a centralized approach for this conversion is better than the dencentralized concept in the dish design. Second, the (heavy) engine is part of the moving structure, which requires a rigid frame and strong tracking system. Furthermore, parabolic mirrors are used instead of flat mirrors and tracking must be dual-axis.

In 2005 Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity. However, as of October 2007 it was unclear whether any progress had been made toward the construction of the 1 MW test plant, which was supposed to come online some time in 2007.

Fresnel reflectors

A linear Fresnel reflector power plant uses a series of long, narrow, shallow-curvature (or even flat) mirrors to focus light onto one or more linear receivers positioned above the mirrors. On top of the receiver a small parabolic mirror can be attached for further focusing the light. These systems aim to offer lower overall costs by sharing a receiver between several mirrors (as compared with trough and dish concepts), while still using the simple line-focus geometry with one axis for tracking. This is similar to the trough design (and different from central towers and dishes with dual-axis). The receiver is stationary and so fluid couplings are not required (as in troughs and dishes). The mirrors also do not need to support the receiver, so they are structurally simpler. When suitable aiming strategies are used (mirrors aimed at different receivers at different times of day), this can allow a denser packing of mirrors on available land area.

Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo). Based on the Australian prototype, a 177MW plant is proposed near San Luis Obispo in California and will be built by Ausra

A Multi-Tower Solar Array (MTSA) concept, that uses a point-focus Fresnel reflector idea, has also been developed, but has not yet been prototyped.

Fresnel lenses

Prototypes of Fresnel lens concentrators have been produced for the collection of thermal energy by International Automated Systems No full-scale thermal systems using Fresnel lenses are known to be in operation, although products incorporating Fresnel lenses in conjunction with photovoltaic cells are already available.

The advantage of this design is that lenses are cheaper than mirrors. Furthermore, if a material is chosen that has some flexibility, then a less rigid frame is required to withstand wind load.

MicroCSP

MicroCSP collectors are based on the designs used in traditional Concentrating Solar Power systems such as power tower and parabolic troughs but are typically smaller and operate at lower thermal temperatures usually below 600 degrees F. These systems are designed for modular or rooftop installation where they are easy to protect from high winds, snow and humid deployments .

Heat storage

Heat storage allows a solar thermal plant to produce energy at night or overcast days. The advantage is that the power generation becomes reliable and the utility can sell this higher quality product for higher prices. Also, the utilization of the generator is higher which reduces cost. The general principle is to transfer the heat to a substance which can hold the heat with a high energy density.

A recent heat transfer material that has been successfully demonstrated is molten salt. Salt contains sodium, and sodium is a metal with a high heat capacity.

The PS10 solar power tower stores heat in tanks as pressurized steam at 50 bar and 285C. The steam condenses and flashes back to steam, when pressure is lowered. Storage is for one hour. It is suggested that longer storage is possible, but that has not been proven yet in an existing power plant.

The proposed power plant in Cloncurry Australia will store heat in purified graphite. The plant has a power tower design. The graphite is located on top of the tower. Heat from the heliostats goes directly to the storage. Heat for energy production is drawn from the graphite. This simplifies the design.

Conversion rates from solar energy to electrical energy

Of all of these technologies the solar dish/stirling engine has the highest energy efficiency. A single solar dish-Stirling engine installed at Sandia National Laboratories National Solar Thermal Test Facility produces as much as 25 kW of electricity, with a conversion efficiency of 40.7%.

Solar parabolic trough plants have been built with efficiencies of about 20%. Fresnel reflectors have an efficiency that is slightly lower (but this is compensated by the denser packing).

The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the radiation (1 kW/m²; see Solar power for a discussion) that falls on its 4,500 acres (18.2 km²). For the 50 MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6%

Furthermore, efficiency does not directly relate to cost: on calculating total cost, both efficiency and the cost of construction and maintenance should be taken into account.

Levelized cost

Since a solar power plant does not use any fuel, the cost consists mostly of capital cost and some operational and maintenance cost. If lifetime of the plant and interest rate is known, then the cost per kWh can be calculated. This is called the levelized cost.

The first step in the calculation is to get rid of the large numbers (millions, mega, etc.). An important number is the investment for the production of 1 kWh in a year. Example, the fact sheet of the Andasol 1 project shows a total investment of 310 million euros for a production of 179 GWh a year. Since 179 GWh is 179 million kWh, the investment per kWh year production is 310 / 179 = 1.73 euro. Another example is Cloncurry solar power station in Australia. It produces 30 million kWh a year for the price of 31 million Australian dollars. So, this price is 1.03 Australian dollar for the production of 1 kWh in a year. This is significantly cheaper than Andasol 1, which can partially be explained by the higher radiation in Cloncurry over Spain.The investment per kwh cost for one year should not be confused with the cost per kwh over the complete lifetime of such a plant.

In most cases the capacity is specified for a power plant (for instance Andasol 1 has a capacity of 50MW). This number is not suitable for comparison, because the utilization can differ. If a solar power plant has heat storage, then it produces also after sunset, which means that it produces more energy for a solar power plant without heat storage with the same capacity.

Although the investment for one kWh year production is suitable for comparing the price of different solar power plants, it doesn't give the price per kWh yet. The way of financing has a great influence on the final price. If the technology is proven, an interest rate of 7% should be possible. However, for a new technology investors want a much higher rate to compensate for the higher risk. This has a significant negative effect on the price per kWh. Independent of the way of financing, there is always a linear relation between the investment per kWh production in a year and the price for 1 kWh (before adding operational and maintenance cost). In other words, if by enhancements of the technology the investments drop by 20%, then the price per kWh also drops by 20%. If a way of financing is assumed where the money is borrowed and repaid every year, in such way that the debt and interest decreases, then the following formula can be used to calculate the division factor: (1 - (1 + interest / 100) ^ -lifetime) / (interest / 100). For a lifetime of 25 years and an interest rate of 7%, the division number is 11.65. For example, the investment of Andasol 1 was 1.73 euro, divided by 11.65 results in a price of 0.15 euro per kWh. If one cent operation and maintenance cost is added, then the levelized cost is 0.16 euro. Other ways of financing, different way of debt repayment, different lifetime expectation, different interest rate, may lead to a significantly different number.

If the cost per kWh may follow the inflation, then the inflation rate can be added to the interest rate. If an investor puts his money on the bank for 7%, then he is not compensated for inflation. However, if the cost per kWh is raised with inflation, then he is compensated and he can add 2% (a normal inflation rate) to his return. The Andasol 1 plant has a guaranteed feed-in tariff of 0.21 euro for 25 years. If this number is fixed, it should be realized that after 25 years with 2% inflation, 0.21 euro will have a value comparable with 0.13 euro now.

Finally, there is some gap between the first investment and the first production of electricity. This increases the investment with the interest over the period that the plant is not active yet. The modular solar dish (but also solar photovoltaic and wind power) have the advantage that electricity production starts after first construction.

Given the fact that solar thermal power is reliable, can deliver peak load and does not cause pollution, a price of 10 dollarcent starts to become competitive. Although a price of 6 dollarcent has been claimed With some operational cost a simple target is 1 dollar (or lower) investment for 1 kWh production in a year.

History

A description of the history can be found on the site of the company Ausra

Modern use of solar technology started after the Oil crisis in 1973 and 1979. Which resulted in the build of SEGS in California and some smaller projects.

Due to low energy prices after 1990, no new commercial plans were made, but some research was still continued.

New commercial plans were made from 2005 and onwards.

See also

References

External links



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Last updated on Wednesday March 12, 2008 at 02:45:56 PDT (GMT -0700)
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