Concentrating the SunThe sun’s energy is diffuse. It is always first collected and then concentrated. In utility-scale systems, solar energy tends to be concentrated at the collectors and to much higher levels. In residential systems, we tend to use solar panels to collect the sun’s energy and some other means to concentrate the energy. This is why we tend to refer to utility systems as “concentrators” and residential panels as “collectors.” Utility-scale systems also tend to be sited in the best locations and often useheliostats to track the sun’s movement for a slight additional efficiency, while residential panels do not have sufficient economies of scale.
Solar thermal systems concentrate the energy in thermal storage systems. PV panel systems do this electrically (but not always with storage). When installing solar PV panels, you must initially decide if you want a grid-tied system where you can sell power back to the utility but keep no on-site storage or if you want an independent system that incorporates a battery or other backup (off-grid). There are also hybrid systems that use a combination of PV panels, battery backup and emergency diesel generators.
PV panels are used when what we want is electricity. There are two major types, silicone crystal and thin-film. Silicone panels produce DC power that must then be run through an inverter to give us the AC power of our electrical grid. The trend is to have AC inverters built into the panels for fewer parts and a simpler installation. For commercial PV panels, the maximum efficiency at the moment is 29%, using present silicone-based technology. Commercially available panels can be purchased with about 8 to 20% efficiency. These are also the most expensive type of solar panels, presently costing between $1 to $3/watt of rated power. Presently, prices are depressed due to a glut of panels on the market, but this is not expected to continue into the future.
Solar thermal panels (collectors) are used when what we want is heat. There are 4 major types: “flat panel collectors,” “vacuum tube collectors,” solar trough panels, and solar air panels. The first three types use a medium like water to transfer the sun’s heat from where it is collected to a central area where the heat can be stored and concentrated. Most of these types tend to be active systems that use pumps to move a working fluid. There are also passive designs that use thermosyphoning.
Flat panel collectors rely upon the sun heating a target which then transfers the heat to pipes that run to a central storage.
Solar trough panels are similar to a utility scale system with a reflector behind a pipe that focuses the sun’s power to a central pipe. The heat is then transferred to central storage. These were used decades ago and mostly fell out of favor due to their complexity. This panel type gains efficiency with solar tracking using a heliostat. A double parabolic reflector behind them obviates the need for solar tracking (here are photos showing the parabolic reflector shape). A vacuum tube covering would also improve efficiency and new research has recently provided both of these improvements.
Vacuum tube collectors now most commonly use a phase change material (eg ether) in a pipe that heats up, changes to a gas and transfers the heat to a manifold where piping collects it and takes the heat to central storage. Older types were similar to the solar trough panels, with the pipes surrounded by the double wall vacuum tubes. Vacuum tubes virtually eliminate heat loss through conduction and convection, but flat panels do lose some heat. In colder climates, this can sometimes be useful to melt snow loads in winter months. Solar panels can overheat, so flat panels are also better in warmer climates, possibly without glazing.
Advantages of Solar Thermal SystemsAny of these thermal panels are more efficient (60 to 80%) and cheaper than PV panels. Infra-red radiation (heat waves) carry more energy than the visible light radiation upon which most of photovoltaics is dependent. Efficiency is also gained by not having to transform the light energy to electrical energy. Vacuum tube collectors even work on cloudy days and in cold wind and weather. The systems have built-in energy storage for cloudy days and nights. The systems cost less and give back more, so ROI or payback is more favorable. These systems that produce heat can efficiently be used to produce air conditioning using the less conventional absorption systems. Also, they could be used to produce electricity by using that heat in an organic Rankine cycle, a stirling engine to operate thermoelectric devices, and in other ways, although this substantially reduces efficiency and increases cost.