Moving away from the Sun Belt locations with their near-perfect direct normal irradiance (DNI), CSP Today’s Andrew Williams explores the advantages of small-scale and modular CSP options for temperate regions.
By Andrew Williams, UK correspondent
Compensating for size, several technologies enable developers of smaller scale CSP systems to capture waste heat and convert it into a cost-effective source of electricity.
Several smaller-scale and modular CSP systems use an Organic Rankine Cycle (ORC) to recover heat from low-temperature sources.
A good example is UK-based Freepower’s ORC Turbine Generator, a closed-cycle electrical power-generation system driven by external heat sources. It comprises a generator, directly coupled to a multi-stage turbine driven by high-pressure hot gas (the working fluid), which is heated up and vaporised by the waste heat source before driving the turbine.
Two US products also employ ORC technology. Sopogy’s MicroCSP system is designed on a low temperature, low pressure scheme, whereas Trimodal’s LTPC engine is a positive displacement device capable of using heat sources as low as 180F / 82 degrees Celsius.
“A positive displacement device is far more efficient and therefore capable of producing mechanical energy at a much lower pressure,” says Marty Johnson, President of Trimodal Group.
France-based Heat2Power’s system does not use an ORC, instead using air as the working medium. It sees this as an important advantage for CSP since it makes it possible to run in an open thermodynamic cycle, aspirating ambient air and exhausting hot air, thus eliminating dry or liquid cooling requirements and saving on cost and water consumption.
Many current offerings are relatively small-scale, which can be an advantage in some situations. For example, the Freepower system can be located at the point of energy consumption (say, alongside rooftop solar-collectors), removing the need for a grid and eliminating distribution costs.
Other systems, such as Heat2Power’s and Sopogy’s, are modular, opening up the possibility of building them up to utility-scale. However, the ideal scale is likely to vary between applications.
“In the case of solar absorption cooling, the technology is ideally [suited] to rooftop-installations, [whereas] for process heat system sizes can be as small as several collectors to several hundred collectors. In power generation, the technology is best suited to utility-scale ground-mounted applications,” says Darren Kimura, President & CEO of Sopogy.
Trimodal's system differs because it is designed for commercial or utility-scale. Their current unit is a 100kw system, sufficient to power about 60-80 ‘US-sized’ homes. They have recently finished engineering a second 250kw unit and expect to rapidly scale-up to larger-sized 250kw, 500kw, 1MW, 2.5MW, and 5MW modules.
“The technology could potentially be scaled to volumes above 5MW, but we feel that it will be most efficient to construct and install in those sizes,” says Johnson.
Although initially slated for automotive applications, Heat2Power soon considered its concept for other uses and are now paying ‘strong attention’ to the CSP sector.
“It makes more sense to run a heat engine 12-15 hours per day on concentrated sunlight that it does for about an hour per day in a car”, says Managing Director, Randolph Toom.
“We see several target-markets. But as the technology [is] small, it fills the gap between Stirling engines and steam/gas turbines. This gap will become more and more important in decentralized power-generation, and in countries where the grid is not yet available or in poor condition, it can become a life-changer”, he adds.
Sopogy’s focus is to expand into new and emerging solar power markets between 1-50mw and substantially reduce costs. However, given the larger size of their system, Trimodal’s target CSP markets are primarily in large commercial and utility-scale solar-thermal projects.
Is this the breakthrough technology that could drive down cooling costs and boost efficiency for utility-scale projects? “Most definitely”, says Johnson, “we would be able to add capacity from their waste heat and have a big impact on cooling costs.”
However, size may not be the only important factor in driving down costs. As Toom highlights, generators that run 24 hours a day are great for rapid returns on investment.
“In CSP applications, we see rooftops becoming more important because energy reflected by mirrors isn’t heating up the building, which in turn requires less cooling capacity. In my opinion, factories, shopping malls and large office buildings in sunny countries should always be equipped with CSP”, he says.
However, does the emergence of waste heat capture technology undermine current views that the optimal size for CSP is upward of 100mw? At this stage it’s difficult to tell, since the optimal sizing of projects depends on many factors, including grid-access and the availability of land and local water resources.
“I think that at the end of the day the question will not be ‘what is the optimal size of CSP?’ but rather ‘what size CSP do I want?’ Since the market in not yet mature, and neither are some CSP technologies, we will see the question coming back and being answered differently according to local conditions, politics, presence of a reliable grid, local cost of maintenance and so on,” says Toom.
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