Combined heat and power generation has a lot in common with CSP, leading some researchers to wonder whether the two could profitably go together.
By Jason Deign in Barcelona
Engineer Ben Davies-Nash is warming to CSP. Which would not be surprising if he worked for a solar energy business. But his company, Houston-based Dresser-Rand, is nothing of the sort.
It is among the largest global suppliers of custom-engineered rotating equipment for the oil, gas, petrochemical and process industries, and Davies-Nash’s particular field of expertise is combined heat and power (CHP) generation. So why the interest in CSP?
“Many of the principles of the CSP field are common with those of CHP,” he says. “It would seem to me there are multiple crossovers.
“For instance, when the sun shines less brightly the solar array has to regulate down as the thermal input is reduced, with a consequential drop in efficiency, increased equipment duty cycles, fluctuations in output.
“In these instances the thermal ‘gap’ could be made up by the cycling of CHP units take up the base load of power supply and provide supplemental heat to the thermal cycle, to reduce the amount the CSP has to regulate down by.”
Davies-Nash envisages a setup where the thermal fluid of a CSP plant, when not being heated by the sun, could be warmed by the jacket water and back-end exhaust heat from a CHP system set to run the turbines at part or full power.
But whether it would be worthwhile in practice would depend on relative gas and electricity export prices of the site.
Also, says Davies-Nash, “the reason why miniature steam turbines and Stirling engines aren’t fitted as standard to the waste heat streams is because they are traditionally cost prohibitive.
Whether the payback from the electricity gained will justify the cost of integrating CHP into a CSP site is currently unknown, he says, as are the answers to a number of other critical questions.
Certainly, this is not a notion that seems to have occurred to many others. Industrial groups contacted by CSP Today, such as Alstom and Bechtel, were not aware of activity in this area.
And Cristina Lamelas, spokeswoman for Acciona Energy, said her company had not carried out any research in this area: “We don’t tend to do studies on things we aren’t planning to do.”
Meanwhile Anna Higueras, of the Spanish CHP industry body Cogen Spain, says her organisation: “Has no definite study on combining cogeneration and solar thermal energy.
“Since it represents a technology which has little operational background the CHP market has not considered starting these kinds of studies.
Tom Fern, communication manager for the Combined Heat and Power Association, also says putting CSP and CHP together “is news to me,” but adds the notion could have benefits for both industries.
“It’s emergent stuff,” he says. “From a general perspective, CHP is not appreciated by government as much as it could be. The attitude is, ‘it’s gas-fired, so it’s not as interesting as offshore wind, for example.’ But it could also be used with renewable energy.”
However, a challenge that some analysts foresee is that in a CSP-CHP setup one of the main benefits of cogeneration—being able to provide low-temperature heat for other uses—would be lost.
Jenny Chase, solar insight manager for Bloomberg New Energy Finance, says: “The immediate problem I can see is the waste heat from a solar thermal electricity generation plant would be in the middle of the desert, probably a long way from residential or commercial demand for heat.
“A lot of the CHP in existence now is supplying waste heat from power plants to heat homes, but there is not likely to be demand for home heating close to solar thermal plants.”
Nevertheless, as reported previously in CSP Today, some developers are now looking at industrial applications for waste solar thermal heat.
Chase’s colleague Nathaniel Bullard, North American solar lead analyst, adds: “More likely that a technology provider would build a solar thermal steam system specific for the heat part of CHP, and optimise accordingly.”
And there is an application where one developer, at least, is excited about putting CSP and CHP together.
Luigi Crema, scientific coordinator of a project called Digespo at the Fondazione Bruno Kessler Renewable Energies Environmental Technologies Unit in Trentino, Italy, is working on the idea of a micro-CSP-powered CHP unit to provide electricity and heating for single buildings.
Initially producing 1-2 kWe and 4-8 kWth, rising to 3 kWe and 12 kWth in production, the Digespo units are “based on a vision of a society which will be more multi-layered, using renewable technologies at a distributed level,” Crema says.
He plans to have a prototype operating at 65% to 70% efficiency in place at the Hilton Hotel in Malta next June. While not quite the utility-scale application that Davies-Nash imagines, it may at least go some way towards raising awareness of the potential for using CHP alongside CSP.
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