Rooftop CSP: Greening the cities
11 February 2010
Green building certification along with a host of demand-drivers may provide micro-CSP with critical market leverage.
By Rajesh Chhabara
Additional reporting by Rikki Stancich
Green building certification is gaining traction in many countries as a means of energy management.
With buildings accounting for some 74 percent of electricity use in the United States, it is hardly surprising that energy efficiency and use of renewable energy have become key criteria in green building certification standards such as US Green Building Council’s LEED certification and UK’s BREEAM certification.
As a relatively new product to market, micro CSP opens up new opportunities for buildings – green or otherwise – to efficiently generate on-site renewable energy.
A portable and scalable on-site solar energy solution, micro CSP can be used to generate electricity, as well as to provide heating and cooling. Suitable for generating energy in the range of 75 KW to 20 MW, micro CSP’s size means the system can be installed in a small area – including on the rooftop of a building.
“A small rooftop, for example the roof top of a large box store such as Wal-Mart, is sufficient to install the micro CSP system,” says Darren Kimura, chief executive of Honolulu-based Sopogy Inc, which has developed a range of micro CSP solutions.
In Sopogy’s case, the micro version runs of an organic ranking cycle (ORC), which instead of using steam, uses the temperature difference between fluids in a closed loop. Unlike large-scale CSP, which requires desert-like conditions of perfect direct normal irradiation, Sopogy’s micro CSP can operate in or near cities.
This is because the ORC is less vulnerable than plants running off a steam turbine, where a cloud passing overhead can cause the steam temperature to drop and the system to shut down.
France-based technology developer, heat2power, has come up with its own version of micro CSP, which uses CSP to power traditional cylinder engines rather than steam turbines or Stirling engines.
The concept is similar to that of a standard combustion engine, except the engine is powered by compressed air, using an external heat source (air heated on a solar receiver to around 900 – 1200°C) rather than from fuel creating an internal combustion.
Using a point concentrating solar array to reflect light and heat to a central receiver tower, the heat2power model is scalable and can utilise anything from a single cylinder 0.5 litre engine to large marine engines based on marine engine blocks, generating power from 10KW – 5MW.
An added advantage is that the engine runs in an ‘open cycle’ and therefore does not require cooling towers or expensive dry cooling used for condensing steam.
Like Sopogy’s solution, heat2power’s captures the waste heat, which can then be used for heating, cooling and desalination.
“Factory roof tops are the ideal place for small scale CSP. It doesn’t require purchase of land and the shade of the mirrors lowers the cooling requirements of the building underneath“ says Randolph Toom, managing director of heat2power.
This is significant, given that in many regions air-conditioning can account for more than 50 percent of the electrical usage in buildings over a six-month period.
Greening the city, not breaking the bank
Micro CSP can help take green building standards to the next level, given that the system reduces the use of fossil fuel to heat, cool and power buildings.
Currently, LEED and other green certification systems do not require specific renewable technology, nor do they standardise energy systems.
However, compared to other options on the table, micro CSP promises to significantly help builders achieve points for certification.
LEED certification currently allows up to seven points for on-site renewable energy, out of the total 110 points possible. BREEAM, UK’s green building certification standard also awards points for the use of low carbon energy and renewable energy.
Here, micro CSP has an advantage over other similar-scale solar products such as photovoltaic panels.
“More carbon is emitted during manufacturing of PV panels than in making micro CSP system,” says Rajan Kasetti, chief executive of California-based Terrafore Inc, a renewable energy consulting and technology firm.
Kasetti also notes that while PV technology may be good for meeting 5-10 percent of a large building’s energy needs, micro CSP on the rooftop can provide 30-40 percent of the energy needed.
Builders, who often cite increased cost of adding renewable energy system as a reason for avoiding them, will find micro CSP’s shorter payback period very attractive. Kimura estimates the payback to range from three to seven years, depending on project size.
“If you use photovoltaic, it does increase the cost. But solar thermal cooling does not. It actually makes the overall building more efficient,” says Kimura.
This is demonstrated via an efficiency comparison in the case of air-conditioning. Photovoltaic efficiency is around 9 percent, compared to solar thermal efficiency, which can be as high as 40 percent.
Toom similarly notes that the return on investment on the heat2power CSP solution is much faster than for other solar options.
“The cost is roughly €300/kw (US$407/kw) for a heat2power engine, compared with around €2000/kw (US$2,712/kw) for a Stirling engine or €1000/kw (US$1,356/kw) for a steam turbine,” he says.
Compared to other small-scale renewable energy options, micro CSP is a more efficient and, given its capacity for storage, a more stable option.
“Using micro CSP, heat can be stored which then can be used to generate power or steam when the load is higher in the evening or when the sky is cloudy,” points out Kasetti. “CSP is also much more stable than the PV which is subject to fluctuations,” he adds.
Support policies such as feed-in tariffs will play an important role in increasing the market penetration of distributed generation systems such as micro CSP.
“Solar energy proliferation has happened in countries where there are good feed-in tariffs as in some parts of Europe,” notes Kasetti.
The UK government recently introduced a feed-in tariff scheme to encourage households and local communities to set up small-scale clean energy generation of up to 5 MW.
The UK’s scheme enables households to earn money by installing low carbon electricity generation. The government also plans to introduce renewable heat incentive for installing micro combined heat and power next year.
Such policies, if adopted elsewhere, will undoubtedly boost renewable micro-generation technologies such as micro-CSP.
Increasing demand for green buildings from sustainability-committed multinational companies will also expand the number of micro CSP installations.
“The builder has a marketing advantage if the building is fitted with on-site renewable energy,” says Kasetti.
But Kimura estimates it will take about five years before micro CSP becomes commonplace in buildings.
“Most other CSP companies are selling utility scale power projects. For us, the challenge is in educating people about the benefits of micro CSP and explaining that CSP is not just for the middle of desert and that it can actually be used for generating steam and cooling as well,” he concludes.
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