London, UK– Last November saw the official inauguration of a novel 272 kW solar cogeneration project in California. At a ceremony attended by former British Prime Minister Tony Blair at the Sonoma Wine Company, the plant was hailed the first commercial-scale installation of its kind, combining proven photovoltaic and solar thermal technologies.
The work of Cogenra Solar, a provider of distributed solar cogeneration systems and renewable energy service solutions, the project now supplies renewable heat and electricity to the winery from a single solar array, which Cogenra CEO Gilad Almogy describes as “an important move towards more affordable and efficient utilisation of solar energy.”
“This is a very significant milestone for Cogenra as we bring our first project on-line in California’s wine country and toast to a bright future with solar cogeneration. [The] technology merges the best photovoltaic and solar thermal technologies to meet two valuable industrial needs: low-cost heat and electricity. Our solution produces five times more energy and three times the greenhouse gas reductions over traditional solar offerings,” he said.
They are impressive claims. So, why is the technology relatively undeveloped, compared to PV solar and PV thermal on their own? And, moreover, why is Cogenra among only a handful of companies worldwide to have focused on this potentially highly lucrative business of reaping solar’s thermal and electric potential in a single, composite component?
It is an area of solar development that Almogy believes is set to grow exponentially. ‘The market is so large, and the technology so new that there is little by way of competition,’ he told REW. ‘We just have to reach out to customers. We have to get people thinking not just about renewable electricity, but holistically about renewable energy, encompassing both electricity and heat.’ The economic and environmental benefits speak for themselves, he says.
Ordinary PV systems convert only 15-20 percent of the sun’s energy into electricity. Conventional solar hot water (SHW) systems miss an opportunity to generate electricity, which often has four times the value of replacement natural gas (typically the cheapest fuel used to heat water).
The thermodynamic advantages of exploiting PV and SHW together in solar cogeneration are similar to those of exploiting combined heat and power (CHP) in a conventional gas-fired cogeneration plant. CHP is more efficient than grid electricity because the heat of a CHP plant is utilised rather than wasted. However, such energy is not renewable power. CHP emits a wide range of air pollutants, which subject the owner to increasingly strict regulatory burdens and possible fines. Pollutants can include nitric oxides, volatile organic compounds (VOCs), carbon monoxide, etc.
A PV/T installation (Source: Cogenra Solar)
Further, CHP operating costs are vulnerable to increasing natural gas prices. On the other hand, solar cogeneration surpasses CHP because it generates renewable energy, hedges volatile gas prices, and emits no pollutants. In addition, solar cogen can deliver up to five times more renewable energy and nearly twice the overall economic value of a PV system of equivalent size.
Yet it was not until the mid-1990s that work began on developing prototypes for PV/thermal combination systems. An entire decade later in January 2005 the International Energy Agency (IEA) initiated Task 35 — ‘PV/Thermal Solar Systems’ — as part of its Solar Heating and Cooling (SHC) Programme.
The objectives of this three-year research programme were principally to help develop and market commercially competitive PV/thermal solar systems. Efforts soon divided into two camps, with most research in Europe focused on combining PV with liquid-filled collectors, while developers elsewhere combined PV with an air -charged collector medium.
Toronto-based Conserval Engineering is in this second category, developing a PV/T system based on its SolarWall air heating technology. The company’s early efforts resulted in two of the world’s most notable and widely publicised systems. First came the Beijing Olympic Village, using a PV/T system sized for 10 kW of electricity and 20 kW of thermal heating energy. A second system at the John Molson School of Business at Concordia University in Montreal featured a larger, 100 kW system, producing 24.5 kW of electricity and more than 75 kW of thermal heating.
At the heart of PV/T technology is the principle that solar radiation raises the temperature of PV modules, reducing their electrical efficiency. Typical PV modules lose about 0.5% of output for every 1°C the temperature rises. The beauty of combining PV and solar thermal is that cooling the PV modules raises efficiency. By proper circulation of a fluid with a low inlet temperature, heat can be extracted from the PV modules, maintaining the electrical efficiency at a satisfactory value. The extracted heat energy can be utilised in several ways, increasing the total energy output of the system.
Hybrid PV/T systems are consequently suitable for those applications subjected to high solar radiation and high ambient temperature. A few companies have further developed the idea into hybrid solar collectors that combine PV cells with a solar thermal collector in a single device.
A complete system typically incorporates components such as hot water storage tanks, heat exchangers, piping, controllers, inverters, wiring and heat pumps. Together, they help PV/T modules to generate more energy per unit surface area than comparable side-by-side PV panels and solar thermal collectors, and at potentially lower production and installation costs. Crucially, in the case of buildings, PV/T modules share the aesthetic advantage of PV panels. High efficiency per unit surface area makes PV/T particularly well-suited for applications with both heat and power demand and buildings where available roof space is limited.
Advocates of solar hybrid technology claim that PV efficiency has been tested at as high as 28% while simultaneously producing a constant supply of water at 60-70°C. A PV/T hybrid panel stabilised at an average of 45°C will produce roughly 20% more (electrical) output over a 12-month period than a standalone PV system with the same peak output.
Solar cogeneration’s claimed capability to potentially capture and convert up to 80% of the sun’s incident energy into either electricity or hot water within a single module also makes this integration of PV and solar hot water technology by far the most cost effective solar energy solution available for commercial and industrial-scale customers. Solar thermal and solar photovoltaic technologies can furthermore also be accommodated in space that would ordinarily be insufficient for both.
In this way, solar cogeneration can extract almost twice the energy value per unit area as a conventional solar collection infrastructure. As a comparable initial investment yields twice the financial return and can qualify for generous PV and SHW incentives, solar cogeneration can thus provide immediate savings to customers over utility rates, says Cogenra.
Similarly, goes the argument, solar cogen’s environmental benefits are correspondingly greater. It represents the most efficient solar power technology, offering by far the fastest payback and cleanest environmental profile available to commercial and industrial customers (see the box panel on the Sonoma Wine case study).
Most air-based systems installed to date have been one-off building projects. Water-based modular systems feature in many more commercial applications. These are typically based on a commercial solar thermal collector where the absorber is modified to integrate solar cells.
The key to the technology’s success lies in optimising the overall performance of the PV/T collector. As noted previously, where mono- or poly-crystalline silicon solar cells are employed, electrical performance is sacrificed as the temperature increases, which means the best performance will be obtained by optimising the system to operate at as low a temperature as possible. Yet in the case of domestic hot water, where the optimum temperature is about 50°C, a higher temperature is required in the absorber. A compromise must be found in designing PV/T collectors where the collector temperature meets all the demands of both thermal and electrical load.
Get the design just right, says Cogenra, and for sites that consume both electricity and hot water, solar cogeneration will always offer a significantly higher return on investment (ROI) and faster payback than any other individual solar technology — PV or SHW — because it delivers significantly more energy and the highest possible value mix of energy types for the same investment.
What’s more, this fundamental advantage is insensitive to the specific assumptions in the financial model, so long as they are applied uniformly across technologies. Put simply, says Cogenra, in many scenarios where a traditional PV installation yields a payback of 10-20 years, solar cogeneration can achieve an ROI in 4-7 years. Additionally, by generating more energy from the same infrastructure, solar cogeneration averts more greenhouse gas (GHG) emissions than PV or SHW in isolation.
PV/T technology can also benefit from a wide range of incentive schemes currently available to developers and owners of renewables projects. In the US states of California and Arizona, for example, solar cogeneration qualifies for generous PV and SHW incentives, peaking last year through a combination of dual state incentives, federal tax incentives (available through 2010 as cash grants), and the two-for-one advantage in delivered energy value. An investment in solar cogeneration could pay for itself in full in less than half the time of PV or SHW alone, its advocates claim.
For a good proportion of its lifetime after ROI, a solar cogeneration system can effectively provide free electricity and hot water, insulating the owner from rising electricity and gas tariffs. Return on investment may be even greater when the extra financial value implicit in this hedge is considered: both carbon legislation and a rise in demand over supply are highly likely over the next three decades. Third-party ownership options such as Heat and Power Purchase Agreements (HPPAs) also offer alternative means for end customers to gain energy savings without upfront expense or ongoing maintenance burdens.
Elsewhere, in the UK for example, PV/T panels may be Microgeneration Certification Scheme (MCS) approved. Newform Energy, which distributes the technology in the UK, says its hybrid panels qualify for the government’s feed-in tariff (FiT) and renewable heat incentive (RHI) schemes.
Newform offers the ANAF H-NRG panel, with a peak electrical capacity of 230 W and thermal capacity of 700-800 W, depending on configuration. Alternatively, the Volther PowerTherm panel can produce 175 W of electricity and 680 W of thermal energy. The Volther PowerVolt is designed with a higher electrical output at 190 W per panel and 460 W thermal. Newform Energy state that these panels can produce up to 30 percent more electricity than an equivalent PV panel at peak irradiance, though annual average output is about 12% higher. A system can therefore be configured to produce the same amount of energy from 16 m² of roof space that conventional photovoltaics and solar thermal would produce from 22 m².
So, what does the future hold for PV/T? Photovoltaic systems (including solar cogeneration as a special class) all rely on semiconductor cells, arranged and encapsulated within modules that convert incident sunlight into electricity. Conversion efficiency from sunlight into DC electricity varies depending on the type of cell. Modules based on silicon cells, the most common type, typically range from 15% for multi-crystalline silicon to 20% for the more expensive mono-crystalline variety. High-end mono-crystalline modules are expected to approach 25% over the next few years but will command a very high price.
Exotic multi-junction devices made from compound semiconductors can exceed 40% conversion efficiency, but are so expensive that currently their only practical application is in satellites. Thin-film devices offer lower efficiencies of 6%-12% at lower cost, but the balance of system, engineering and installation costs can outweigh purchase savings in some scenarios.
At California commercial utility rates, solar cogeneration systems already deliver nearly twice the economic savings of a comparably sized CPV or PV system, says Almogy. PV/T’s total delivered energy potential dramatically changes the economics of solar power much faster than striving only to push down the cost, in his view. Solar cogen can also leverage all the complementary ongoing advances in PV. As PV cells, modules, manufacturing techniques and inverters improve in performance and decline in cost year over year, new solar cogeneration systems benefit in lockstep.
Integrating hot water into the customer’s water systems does add a further cost of as much as 20% of the total installation. Yet even after accounting for these costs and their associated efficiency factors, solar cogeneration still produces 50% more net energy value than PV/CPV per dollar invested, proponents of the various solar thermal hybrid technologies argue.
Solimpeks, a Turkish manufacturer of solar thermal and hybrid collectors, says it expects PV-T to grow at an annual rate of 20% from 2010, driven largely by the domestic market. In northern Europe, for example, Solimpeks says a 25 m² area of collector would typically be enough for both an average home’s hot water and its electrical demand. The hybrid solar heating and electric technology is at the start of an exponential growth curve, and volume adoption is set to take off.
The Sonoma Wine Company is the California North Coast’s largest contract services winery, processing more than 7500 tonnes of grapes, bottling 4 million cases and storing and servicing 65,000 barrels of wine each year. The company is noted as an EPA climate leader since 2005 for its greenhouse gas reductions and sustainable practices.
In addition to its electricity consumption, Sonoma Wine Company uses large amounts of natural gas to heat water for its wine processing, sanitation and barrel services operations. The company sought an on-site renewable energy solution with high environmental benefits that best addressed the facility’s energy mix without requiring a large capital investment.
Cogenra engineered a system and financed 100 percent of a hybrid PV/T installation using its Heat & Power Purchase Agreement (HPPA), under which the developer owns and operates the solar cogeneration modules while Sonoma purchases the thermal energy and electricity generated at guaranteed rates for the duration of the contract.
After an extensive evaluation, the project called for 15 Cogenra ‘SunBase’ modules, supplying a total of 272 kW in electric and thermal output. In less than two months from breaking ground, the solar cogeneration system was complete and delivering energy to support the facility’s winery operations. The solar cogeneration system will displace about 64 MWh and 12,500 therms of natural gas annually.
The solar thermal element is used to heat water to 74°C to fuel the facility’s tank wash and automatic wine barrel washing system. More than 800 barrels undergo three wash cycles per three-person shift, with the water from the last cycle recycled to wash the following barrel. With low energy prices for the lifetime of the 15-year contract, the HPPA serves as a hedge against future rate hikes. As utility rates continue to increase, the Sonoma Wine Company expects to reap more savings.