Several energy sector commentators, writers and journalists have recently questioned the viability of concentrated solar thermal power conversion, even going so far to proclaim its impending demise. Solar thermal power conversion in the USA occurs to the north of 30°N, the latitude of New Orleans and also Cairo, Egypt and Delhi, India. Except that most of the planned solar thermal power conversion in the Middle East and India will occur to the south of 30°N while in Australia it will occur to the north of 30°S. Concentrated solar thermal power conversion outside of the band between 30°N and 30°S is seasonal and less cost competitive than power generated inside that band of latitude.
Recent advances in photovoltaic and concentrated photovoltaic technologies have raised efficiency and lowered the price of a very versatile technology. Low-maintenance PV technology can easily be installed on the roofs, sides and incorporated into the window glass of buildings in mainly small-scale installations in dry and arid locations. While decentralized PV conversion incurs higher cost than buying power from the grid, the PV technology along with flow-battery technology serves a very useful purpose by providing a measure of energy security at locations where power outages occur frequently.
The competition facing concentrated solar thermal power conversion places greater emphasis on evaluating the possible future of the technology while using past development in the field as being part of a learning curve. There is merit to using a parabolic reflector to concentrate solar thermal energy on the metal water pipe to heat water for some application. The ongoing need for hot water assures a continued market for parabolic reflectors. There are locations around the world where water, including seawater will still be available and where sunshine is plentiful.
Low-Grade Solar Heat:
There are many forms of low-grade solar thermal power conversion that allow for the use of organic rankine cycle engines. The list of naturally occurring low-grade heat sources includes ocean thermal energy conversion (OTEC) and solar thermal salt ponds at coastal locations. Energy researchers in India are still testing the potential to generate electric power using the difference in temperature between the warm ocean surface (35°C) at tropical locations and cooler water (15°C) found at much greater depths at the same location.
Salt ponds capture rather the infrared spectrum of light while potable and low-salinity water would reflect the infrared spectrum. The temperature at the bottom of salt ponds can reach temperatures of 65°C to 95°C. Cold ocean currents (25°C) flow along several coastal locations around the world where salt ponds may be developed and include:
- West Coast of Australia
- West Coast of South Africa and Namibia
- West coast of Morocco
- West coast of Northern Chile and Southern Peru
- Northwestern coast of the USA
Compared to OTEC operation, an organic rankine cycle engine will operate at higher efficiency over the difference in temperature between a coastal salt pond and a nearby cold ocean current. A spiral coil of pipe made of corrosion-resistant material would be installed on the bed of the pond to collect heat. The salt pond would preheat water flowing through the spiral pipe at 80-psia after which it would be heated by parabolic reflector technology to 148°C (300°F). The organic Rankine-cycle engines would then operate at their maximum allowable temperature and at the efficiency of most commercially available PV technology. The solar thermal technology would incur lower initial and log-term costs.
Steam-based Solar Thermal Conversion:
It was a natural progression to develop parabolic reflectors from heating water to generating steam and involves unraveling water tube boilers to achieve such a purpose. The sheer size and extent of a water tube boiler that is heated by concentrated solar thermal energy presents numerous challenges that include leakage and heat loss across the installation. There may be an advantage to taking the extensive water tube system out of the parabolic reflectors and replacing them with the combination of heliostats and compact boilers.
In some concentrated solar thermal power installations, it would be possible to convert a coil mono-tube boiler into a tightly wound, pancake shaped, flat spiral mono-tube boiler. In other installations, it may be possible to enlarge the coil boiler to include multiple parallel tubes wound on a large diameter and perhaps including a series of inner (preheat) coils and outer (super-heat) coils. There are tropical locations where it would be possible to use the combination of reflector and heliostat technology to focus concentrated solar thermal energy on to both sides either design of boiler.
The boiler may be housed between panes of transparent sapphire-aluminum-oxide to minimize heat loss. While the heliostat technology collect an equal area of sunlight as parabolic reflectors, the more compact boilers could operate at higher pressure and higher temperature while incurring less heat loss and lower potential for leakage than an extensive water tube system. The installation could generate hotter steam at higher pressure to operate steam engines at higher thermal efficiency.
The most recent developments in steam power conversion involve generating ultra-critical steam at pressure levels in excess of 3000-psia at temperatures exceeding 540°C (1000°F). Water-tube boilers made from special martensite steel can operate at the elevated temperatures and pressures. The range of expanders includes specially built steam turbines and even specially built uniflow piston engines developed by Cyclone Power in the USA that can achieve the thermal efficiency of a diesel engine. Prototype concentrated solar thermal installations heated by heliostat technology have recently appeared.
Alternative Concentrated Solar Thermal Power:
There alternative non-phase change forms of concentrated solar thermal power conversion include several types of air-based and gas-based engines that include Stirling-cycle engines and thermo-acoustic conversion technology. The latter concept involves using heat to generate low-frequency sound waves that drive linear alternators to generate some 50kW of electric power at equivalent efficiency levels of over 40%. Solar-heated Stirling-cycle engines can generate up to equivalent output at marginally lower efficiency and involve a high capital cost.
The thermal power research of several groups located in Spain, Australia, the USA, Canada, Greece, Singapore and South Africa revolve around solar-heated chimney technology. One version of the technology proposed to use chimneys built to a height of 1000m to 1500m while a similar floating tower would be made of fabric and be held aloft by balloon technology. A competing concept proposed to use a tower of lower height in which to use available heat to produce a vortex that could pull an air stream through ground-level turbines. The higher towers would produce powerful updrafts that would pull air through ground level air turbines and propose to generate some 200MW to 300Mw of power.
While it would be possible to combine solar salt pond technology with thermal tower or thermal chimney technology at some locations, most researchers propose to use arrays of solar reflectors to concentrate heat on to the towers. A proof of concept of solar heated tower technology has been demonstrated at Manzanares in Spain and produces up to 50kW of output. The low-cost option would combine the solar salt pond with a vortex engine, however such a combination may be possible only at a few locations around the world despite the small size of the tornado the vortex engine would produce.
Solar Heated Brayton-cycle Engines:
The introduction of engine components made from silicon carbide has raised the efficiency and performance capability of small turbine engines. Silicon carbide can maintain constant mechanical properties up to 1400°C with extraordinarily high thermal conductivity. Ongoing developments related to silicon carbide technology suggest the potential to develop high-temperature heaters for externally heated air turbine engines.
The air heater system downstream of the engine compressor would include a recuperative heat exchanger to recover a large percentage of exhaust heat, a spiral-coil primary air heater made from high-temperature stainless steel and an air super-heater made from reinforced silicon carbide. While a trio of a recuperative heat exchanger, primary heater and super-heater may sustain the operation a micro-turbine engine, groups of such trio's could sustain the operation of a much large engine of much higher output.
The same heliostat heated solar thermal technology can be adapted to turbines that operate on compressed air stored in subterranean caverns instead of on compressed air from engine-driven compressors. In the former case, the solar heated turbines would deliver 50% to 60% greater output due to the absence of the power required by the turbine-driven air compressor. There is potential to combine seasonal compressed air energy storage (CAES) with high-temperature concentrated solar thermal power and the concept may be cost competitive against PV technology in large-scale applications.
Hybrid Solar Thermal Technology:
There are a variety of methods by which to combine concentrated solar thermal power technology with other technologies. The possibilities include:
- Super-heat saturated direct/indirect steam from geothermal wells
- Super-heat steam from a light water nuclear power installation
- Convert water to saturated steam that will be super-heated by combustion
- Preheat air that will be superheated by combustion in a turbine engine
- Use natural gas combustion after sunset and before sunrise
Hybrid technology has the potential to provide a niche for concentrated solar thermal technology in a variety of engine applications that use steam, air of a gas as the working fluid.
Solid-State Thermoelectric Technology:
Present examples of solid-state thermoelectric conversion technology operate at a conversion efficiency of 5%. There is much research underway in the USA and in Western Europe to raise the conversion efficiency to within range of PV power and steam-based technology. Solid-state thermoelectric technology has several applications outside of concentrated solar thermal technology, including being placed on top of a fireplace where it can generate power to recharge batteries or provide interior lighting to a residence or building.
Thermal Energy Storage:
The solar heated salt pond is both a solar collector and thermal energy storage system that stores heat using the latent heat of fusion. Building a transparent cover over a salt pond will minimize heat loss during the over night hours while allowing daytime transmission of the infrared spectrum. There is potential to convert small salt caverns to low-grade heat storage systems (150°C) where heat is pumped in during the day and extracted at night to operate either organic Rankin-cycle engines or air-based thermal chimney engines.
The solar thermal power industry has pioneered the development of heat-of-fusion thermal energy storage systems capable of generating steam and that are based on mixtures of naturally occurring sodium and/or potassium salts. Such materials involve low costs and provide greatly extended useful life expectancies. Research is underway to develop eutectic mixtures of lithium-aluminates capable of generating high-grade superheated steam.
Whereas the very large-scale air-based chimney engines can operate on low-grade heat, small-scale air-based engines operate on very high-grade heat at some 1000°C. Containers made of silicon carbide can certainly store heat-of-fusion materials that melt at that temperature. The list would include eutectic mixtures of the hydrated and non-hydrated hydroxides and oxides as well as the oxides and fluorides of the same metals. A mixture of the oxide and hydroxide of thorium would combine high density with a high level of thermal storage in a relatively compact package.
Solar PV technology has already established several unique niches in the world of small-scale and decentralized power generation. Many forms of solar PV technology already operate free from state subsidy and that trend would likely continue into the future and expand. The dropping cost of PV technology relative to its output make it an attractive option for small-scale applications, with potential for large-scale applications.
Concentrated solar thermal power using parabolic reflector technology also has its own unique market niche such as heating water at numerous locations around the world. Most large buildings around the world have need for heated water, as do many thermal-desalination plants. The same technology can be adapted to low-grade thermal power conversion involving organic Rankin-cycle engines.
The cost of heliostat technology as compared to PV technology will determine as to which technology gains favor in large-scale power generation. There is potential to combine heliostats with a different design to boiler to raise thermal efficiency and possibly reduce capital cost. In some nations, it may be possible to desalinate seawater using the exhaust steam from steam-based solar thermal power installations.
The ability to combine concentrated solar thermal power conversion with a related technology in a hybrid system offers to create a unique niche for such technology. Combining seasonal compressed air energy storage technology (CAES) with concentrated solar thermal power technology can allow the hybrid technology to generate greater output from the same number of reflectors during the summer months, when demand for power soars in many nations. The CAES hybrid concept can include a combined-cycle thermal technology that involves both Brayton and Rankine-cycle engines to raise power output and overall thermal efficiency while operating on concentrated solar thermal power. Exhaust heat from the Rankine-cycle engine may be used to operate a thermal desalination plant.
The option to apply hybrid technology and combined-cycle technology to concentrated solar thermal power conversion offers to create a viable niche for an expanded version technology. Such technology may best operate in locations that have the following characteristics:
- Excess power generation during winter
- Excessive demand for power during summer
- Potential to introduce (seasonal) CAES technology
- Location is near an oceanic coast (thermal desalination)
- Optimal latitude between 35°N and 35°S
- Application is of large-scale
Despite the increasingly competitive nature of PV technology, there may be scope to enhance the versatility and competitiveness of concentrated solar thermal technology. While the 2-solar technologies have their respective unique niches, the evolving competition between them will create unique market applications for each technology. Both technologies are likely to evolve and develop further over the years ahead as their respective market niches evolve and develop.