Unused ‘waste’ heat is the enemy of cogeneration, yet solar photovoltaic installations tend to discard large quantities of heat. Could a solar-cogen hybrid be the answer? Dr Gilad Almogy describes the technology and some early successes.

The solar cogeneration system installed at Sonoma Wine Company

Like conventioal CHP, solar cogeneration is ideal for facilities that use significant amounts of hot water on a daily basis. Traditional CHP systems heat water with waste heat created from natural gas turbines during electricity production. Solar cogeneration works in a similar fashion but without using non-renewable resources.

Solar cogeneration can therefore be implemented by the same types of industries that are accustomed to traditional cogeneration, including facilities such as hospitals and college campuses.

Solar cogeneration’s advantages over traditional cogeneration are reduced greenhouse gas emissions and stable energy prices. The availability of solar energy also coincides with peak demand hours.


Solar cogeneration – also known as solar cogen or hybrid solar – combines proven photovoltaic (PV) and solar thermal technologies in a single system to deliver both electricity and hot water.

Despite years of incremental improvements, standard PV panels only capture about 15–20% of the sun’s energy. The remaining 80–85% is discarded as waste heat. With solar cogeneration, this waste heat is used to heat water, raising the efficiency of the system to as much as 75%.

By tapping into this lost resource, solar cogen eliminates greenhouse gas emissions at nearly three times the rate of traditional PV. The process also reduces local volatile organic compounds (VOC) and nitric oxide and nitrogen dioxide (NOx) emissions from burning natural gas. In some air quality districts, avoiding VOC and NOx pollution can lift regulatory burdens and prevent fines.

Lifecycle greenhouse gas emissions from solar cogen systems are estimated to be about 50–60% lower than from traditional PV or solar hot water (SHW) systems. Solar cogen also offers payback times that are typically one half to two thirds of those of standalone PV or SHW systems, ranging between three and five years.


Solar cogeneration modules consist of proven off-the-shelf components that can be quickly assembled on site – either on the ground or on the roof – and are designed to seamlessly integrate with existing hot water equipment.

Water is heated through a closed-loop heat exchanger to about 70°C. According to the facility’s needs, the water is used immediately, fed into boilers to be heated to higher temperatures, or temporarily stored and applied during non-sunny hours. The electricity generated feeds directly to the facility’s existing power supply.

Dual heat and electricity benefits make solar cogeneration attractive for a variety of institutional and industrial industries across sectors as diverse as the food processing industry, multi-family residences, healthcare institutions and spa and fitness facilities – in short, all facilities that use a large amount of hot water and electricity.

The most common applications for hot water from solar cogen include showers, laundry, cleaning, cooking and dishwashing. Institutional sites that support a large community can benefit greatly. The University of Arizona recently decided to install solar cogen arrays on the rooftop of one of its dormitories to fuel laundry and showers. Facebook recently signed up to install a rooftop solar cogen system for the fitness centre at its new corporate campus in Menlo Park, California.

Hotels, hospitals, cafeterias, correctional facilities, multi-family housing complexes can reap significant energy savings by fuelling amenities with solar hot water and electricity. California provides two recent case studies – a winemaker and a manufacturer of agricultural equipment.


Sonoma Wine Company became the flagship solar cogen user when it installed a system last year in Graton, California, to cut the costs of heating water for its routine tank and barrel washing systems.

The company operates bottling, wine processing, and tank and barrel storage services for nearly 3 million cases of wine. The firm has been recognized as an EPA Climate Leader since 2005 for its greenhouse gas reductions, and is known within the wine industry for its sustainable winemaking. 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.

Under a heat and power purchase agreement (HPPA), Cogenra Solar owns and operates the solar cogen modules, while Sonoma Wine Company purchases the thermal energy and electricity generated at guaranteed low rates. Cogenra offered a solution that fits Sonoma Wine Company’s energy demands and environmental goals while involving no upfront capital or maintenance costs.

The Graton facility underwent a site and energy usage evaluation to determine the most effective size of the installation, as well as the engineering requirements for integration with existing natural gas water heaters and electrical equipment. The project called for 15 ground-mounted Cogenra ‘SunBase’ modules, totalling 272 kW in electrical and thermal output.

In less than two months from breaking ground, the system was delivering energy. Erected directly adjacent to the building, the solar arrays occupy less than 1200 m2 of a previously unused plot of land. The system will displace about 64 MWh and 12,500 therms of natural gas annually, a figure that would rise dramatically in areas of greater sun than rainy Northern California.

The solar thermal element will heat water to 74°C to fuel the facility’s tank wash and ‘Tom Beard’ machine, a state-of-the-art automatic wine barrel washing system.

With guaranteed low energy prices over the 15-year contract, Cogenra’s HPPA serves as a hedge against future utility rate hikes. As energy rates continue to increase, the Sonoma Wine Company will reap more savings. Additionally, the HPPA freed the Sonoma Wine Company from all upfront costs and ongoing maintenance responsibilities associated with the facility.

The hybrid solution offers a clean alternative to grid-fed electricity and limits the amount of natural gas burned on site, improving local air quality. The 272 kW installation will eliminate more than 120 tonnes per year of greenhouse gases, the equivalent of removing nearly 25 passenger cars from the road annually.


General Hydroponics (GH), based in Santa Rosa, manufactures indoor agricultural systems and nutrients for flower and vegetable growers worldwide. With clientele spanning research universities, national laboratories and high-tech gardeners, the company’s Sonoma County facilities comprise 21,000 m2 of modern industrial space used for product development and production.

Eight years earlier, GH installed a 101 kW PV system. But much of the company’s utility bill is for natural gas, used in manufacturing various nutrient solutions. GH considered a handful of traditional solar thermal systems, but the forecasted payback period was too long to justify the capital investment.

With solar cogen, GH was able to significantly reduce its spending on energy and get a sub-five year payback. Solar cogen raised GH’s solar electricity capacity, while cutting an estimated 60% of its current natural gas usage via solar hot water.

GH installed Cogenra’s ‘SunDeck’ 75 kW solar cogen system mounted on a standard solar racking system on a rooftop. Consisting of 36 Cogenra PVT (photovoltaic-thermal) receivers, the system produces 15 kW of electricity and about 60 kW of thermal energy. The array occupies about 260 m2 and its lightweight architecture required no special permitting or roof modifications, which is crucial to keeping costs and construction times to a minimum. Fully integrated hydronics, controls and inverter installation provide remote monitoring and performance diagnostics 24/7.

GH’s solar cogen array is expected to offset more than 3500 therms of natural gas use annually. The dual solar hot water and electricity generation qualifies the installation for both solar thermal and solar electric rebates, and tax incentives offered at the state and federal level. In addition to the California Solar Initiative’s photovoltaic incentives, GH is also eligible for the solar hot water rebate offered at the first-tier level of $12.82 per therm of displaced gas in the first year.

With the CSI and federal rebates, GH will experience a return on its investment in less than five years, while immediately hedging against future utility price hikes and gas price volatility. The installation will reduce greenhouse gas emissions by 37 tonnes per year.


Solar cogen is the most efficient and sustainable solar solution for on-site energy generation. Hybrid solutions can produce five times the energy, three times the greenhouse gas reduction and twice the financial savings of conventional solar. Solar cogen also complements traditional cogen by producing electricity and hot water during peak demand hours.

Dr Gilad Almogy is the founder and CEO of Cogenra Solar. Email: info@cogenra.com


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