The Next Solar Frontier: Hybrid PV/Thermal Systems

Already this is starting to occur. The excellence of past inventions and progress in the renewable energy field – refferring specifically to solar now – have created a foundation upon which two main events are based. Firstly, the viable solar technologies which have been developed in the past are starting to be adopted on a broader and more meaningful global scale, across different sectors with ever increasing market penetration. In this context, recall for a moment the history of the microwave, which was invented by US engineer Dr Percy Spencer in 1945, but only began to achieve a noticeable amount of market penetration in the mid-1970s, followed by a rapid adoption rate throughout the 1980s and 1990s. The market penetration rate went from 1% in 1975 in the USA to over 90% in the 1990s. And, as the microwave and other such great inventions are considered to be a necessity of today, so too will solar energy come to be regarded as a necessity of tomorrow.

The second broad trend which is occurring in the solar industry is the prolific amount of work which is going into developing new technological variations to lead the energy market demands of the future.

One of the most promising advances in the solar space today has to do with developing and commercializing hybrid PV/thermal systems. A PV/thermal system is by definition a ‘combination’ system which produces both electricity and heat from one integrated system using the same surface area. Clearly this form of ‘technological convergence’ is advantageous because it enables the clean production of two types of energy in a building – instead of just one – and the new types of climate change legislation that are beginning to be tabled in different countries will include specific targets for ‘renewable heat generation’, in addition to the conventional ‘renewable electricity’ targets’. Also, there are significant technological and financial synergies associated with combining PV with a solar air heating component.

Some History on PV/T Technology and IEA Task 35

In the mid-1990s work began in different parts of the world on developing prototypes for different types of PV/thermal combination systems. The majority of the research in Europe centered on combining PV with liquid collectors. In Canada, Toronto-based Conserval Engineering pursued a different course and began experimenting with a combined PV and air collector system based on their SolarWall air heating technology.

In response to the growing interest in this very promising field, in January 2005 the International Energy Agency (IEA) initiated Task 35 ‘PV/Thermal Solar Systems’ as part of the IEA Solar Heating and Cooling (SHC) Programme. The objectives of this three-year research programme were to help develop and market commercially competitive PV/thermal solar systems. Canada participated in the programme and sponsored several PV/thermal installations, including two of the most notable and publicized systems in the world, the first one being in the Beijing Olympic Village, and the second at the new John Molson School of Business at Concordia University in Montreal, Canada. Both of these hybrid systems combined traditional PV with the SolarWall technology.

Successes in Beijing and Montreal

The PV/thermal systems installed in the Beijing Olympic Village and at Concordia University were both hybrid systems that combined PV with solar air heating. The thermal component offsets the indoor heating load, which is often the largest single source of energy usage for buildings in many parts of the world. As such, there was much excitement surrounding the commercialization of this particular technology because of widespread potential. SolarWall technology is an active solar air heating system with efficiencies up to 80%. It pre-heats the ventilation air that is required in commercial and industrial, thereby offsetting the traditional heating load by 20%–50%. It can also be used for process heat, offering large-scale energy and CO2 reductions, and has been used successfully in over 30 countries around the world.

Traditional SolarWall systems have a metal collector facade that contains thousands of tiny micro-perforations. Outside air is drawn in through these perforations and is heated as it passes into the specially designed air cavity behind. This heated ventilation air – which is now preheated to anywhere from 16–38°C (30–70°F) above ambient on a sunny day – is then ducted into the building’s traditional ventilation system or is distributed throughout the building using fans and ducting.

With the hybrid PV/T technology developed by Conserval the SolarWall system (either wall or roof mounting is possible) becomes in essence a ‘thermal racking’ system for the PV installation. PV modules are mounted onto the SolarWall system and are configured in a way that allows the cooler ambient air to pass behind each PV panel in a uniform and controlled manner.

Heat generated from each PV module will be transferred from the back of each module to the heating system. Controlling the air flow ensures consistent airflow around each PV module and precisely the same amount of heat is immediately removed from each PV panel without overheating adjacent modules. The excess heat is drawn away from the back of the PV module and is ducted into the building’s Heating, Ventilation and Air Conditioning (HVAC) system, where it offsets the heating load. In the summer the system continues to cool the PV modules, but a bypass damper ensures that the heat is vented if it is not required in the building.

The value proposition of the PV/thermal system is that it quadruples the energy production of a traditional PV system. It also allows the PV component to operate at its peak electrical output and mitigates the degradation of PV cells due to overheating (which is a common problem). And because the two types of energy are produced from one surface area, it also solves the growing problem of "competing roof space," which occurs when a roof is covered completely in standard modules leaving no room for other solar technologies. In essence, a 15% efficient roof is transformed into a 50% efficient roof, which is what convinced the Beijing Olympic Park Developer to upgrade the conventional PV system, intended for the roof of a building in the athlete’s village, to a PV/thermal system.

The 2009 Beijing Olympic Village

The Beijing Olympic PV/T system made headlines because it was the first large-scale PV/T in the world. And, just as new athletic performance standards are established at the Olympics, this system established a new standard because it proved the commercial viability, and therefore the widespread potential, of hybrid technology.

The SolarWall PV/T system was installed on the roof of a central building, which was designed to consume 75% less conventional energy than a traditional Chinese building. The system was sized for 10 kW of PV and 20 kW of heating. The building also featured a traditional wall-mount SolarWall system providing ventilation heating only.

An important consideration with PV/T systems that emerged from the development process is that often PV intstallers and designers have a postive perspective on the hybrid systems but are unsure about the integration of the thermal component into the building’s mechanical system since that is typically outside their area of expertise. HVAC connections are always designed by Conserval.

Natural Resources Canada, working in partnership with the Solar Buildings Network, supported the installation of SolarWall PV/T system at Concordia University’s new business school in downtown Montreal. Measuring 100 kW (25 kW of PV and 75 kW of thermal) and covering an area more than 3000 square feet (278 m²), it is considered to be the only truly building integrated PV/T system in the world. The solar system is the exterior wall, which was seamlessly integrated into the front facade where it spans the top of the high-rise building. It is a beautiful architectural feature, in addition to being a source of two types of clean energy, the hybrid system is expected to have an overall solar energy utilization efficiency of up to 60%. It is projected the electrical generation will be increased by some 5% by forcing fresh air behind the PV panels, thus lowering the temperature. This pre-heated air is supplied to the mechanical system.

Why PV/T? The Technology Perspective

To fully understand why PV/thermal systems inherently make sense in many situations, it is worth recalling a few facts about how photovoltaic systems operate.

Historically, the main drawback to many conventional PV systems is the high initial cost and limited amount of electrical output compared to the solar input – paybacks of several decades are not uncommon, where government subsidies or high electric purchase plans are not available. Photovoltaic modules convert energy into electricity at an efficiency of 8%–15%. What happens to the rest of the sun’s energy that shines on the surface of the panel? Some of it is lost through the reflection off the glass, but the majority of it is converted into heat energy that is not only not captured with conventional systems, but also usually results in lower electrical performance because the heat pools behind the PV modules and raises the operating temperature.

Manufacturers rate the electrical output of modules at 25°C. For every 1°C above 25°C, the electrical output drops by 0.4%–0.5%. A typical rooftop PV array may measure 55–75°C (131-167°F), which means its electrical output would fall by 12%–25% below the name plate rating.

Currently in Europe the trend with PV is towards building-integrated PV installations (BIPV) where the modules become part of the exterior skin of the building. While this is advantageous, there is usually no provision made for removing the heat build-up from the back of the PV modules, even though it is common knowledge that these systems can reach temperatures up to 80°C. To put that in perspective, at those temperatures, the performance loss is such that a 10 kW array becomes a 7 kW array.

Testing of the PV/T system at the National Solar Test Facility (NSTF) in Canada showed the addition of the thermal component to the PV lowers the photovoltaic temperature by 10–20°C (50–68°F), which increases the electrical output by 5%–10%, or an extra 0.5–1 kW for a 10 kW array.

The Economics Perspective

Tests show solar PV/thermal systems can generate four times the energy production from the same surface area for a 25% increase in costs.

The SolarWall PV/thermal tests at NSTF went on to show that the thermal energy was up to 300% greater than the electrical energy. For example, a PB 160 W PV panel actually produced over 700 W of total energy with 540 W of thermal energy making up the difference. As a caveat, it is important to understand that PV/T is a custom engineered solution and therefore the specific electrical and thermal outputs can be varied, based on the energy demands of the building.

The energy gain associated with capturing the ‘waste’ heat energy and using it to offset other energy requirements in a building, combined with the increased performance gains associated with lowering the PV module operating temperature, has been shown to reduce PV paybacks by between one third and one half. This improved return on investment is significant because it helps bring the PV payback into a more standard corporate payback period. As well, in jurisdictions where there is a feed-in-tariff (FIT), the financial return is based on the actual electrical energy produced, not the rated electrical output of the system, so optimizing the PV output is an essential consideration.

The Climate Change Perspective

On a global basis, heating accounts for an estimated 50% of the total energy used in the building sector, and therefore it is one of the largest single sources of CO2 emissions. For example, the operation of buildings is estimated to be responsible for almost 40% of CO2 emissions in the USA and Canada, which clearly highlights why meaningful carbon reductions cannot be met without targeting this use of energy. Already the European Union is starting to establish renewable heat targets because the only way to achieve current CO2 objectives is to drastically begin the process of ‘decarbonizing’ the heating component. This type of legislative framework clearly bodes well for the solar thermal technologies, whether they are used on their own, or combined with PV.

The unit costs to generate renewable heat tends to be substantially less than the unit cost to generate renewable electricity, which is exemplified by the fact that PV/T systems typically cost about 25% more than conventional PV, but can generate up to 300% more useable energy.

The Road Ahead

Clearly, PV/thermal systems represent one of the most promising new technology sectors within the renewable energy space. There are numerous synergies to adding a thermal component to building integrated PV systems, and it is likely that one day in the not-so-distant future, solar systems of all types will be considered the ‘norm’ on buildings of all shapes and sizes in all parts of the world.

The calibre and performance of the first two large-scale commercial SolarWall PV/T projects in Beijing and Montreal are a testament to the technological-excellence of hybrid systems and the mass applicability to buildings in countries around the globe. Like the microwave in the 1970s, the solar heating and solar electric technologies are just at the start of their exponential growth curve, and the mass adoption of them over the next several decades will drastically improve our society and our economy, like all the other great technologies that have gone before them.

Victoria Hollick is vice president of operations at Conserval Engineering.