Solar Thermal Panels, Practical but Not Yet Popular – A Solar Overview

The sexy movie stars of the solar panel business are photovoltaic (PV) panels, but you might not want Hollywood deciding what to put on your roof. My colleague, Glenn Meyers, has discussed how solar PV panels work in a concurrent article. I will not cover that here. But every star has their working relatives. For PV panels, the older working brothers are solar thermal panels and “distant cousins” are solar air panels which I come to at the end.

Concentrating the Sun

The 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 use heliostats to track the sun’s movement for a slight additional efficiency, while residential panels do not have sufficient economies of scale.

An old Heliostat design

Solar thermal panel 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 house.)

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, causing some infamous bankruptcies like the Solyndra bankruptcy and possibly others to come.

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 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. The vacuum tubes virtually eliminate heat loss through conduction and convection. Flat panels lose some heat. 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 Systems

Any of these thermal panels are more efficient (60 to 80%) and cheaper than PV pannels. 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. The 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.

Domestic Energy Use

In most places in the US, heating and cooling costs more (about 50 to 70% of energy expenditures) than electricity for other purposes (about 30 to 50%, not considering heating or air conditioning.) Going to the website of the flat panel pictured above gives the solar panel’s specs. The flat panel collector costs $32/sq ft. The Max BTU (power rating) is 5000BTU/h for 21.7 sq ft where insolation is 1000W/m2/day or (5000/21.7=) 230.41BTU/hr/ft2 or 67.53watts (67.53/$32=) 2.11 watts/$ or $ .47/watt more than 3x cheaper than PV panels for the energy received. Evacuated tube systems are more expensive as would be solar trough panels (unless you make them yourself). Solar thermal panels save more and save more where it counts.

So Why are PV Panels More Popular?

If you are going into the business of selling solar panels, to make more money you need a virtual plug-and-play product. Solar thermal panels are part of a relatively complex system that is not as easy to retrofit into existing homes and businesses. The profit margins on PV are usually higher than solar thermal. PV has more use for utilities and commercially than solar thermal, so there are larger potential markets and profits are higher. You hear more about PV because more companies want to make them, not because they are more useful to residential customers.

Solar Thermal Air panels are probably the most difficult to find commercially. This is because they are so simple to make that they are often a DIY project. They are the cheapest panels with the highest ROI and lowest payback period. A passive solar-designed home might have sufficient glazing and let the sun enter the home directly. Solar air panels are a retrofit that are often installed on an exterior wall of a house and are used to actively heat the air in a house indirectly. At its most basic, a solar air panel is an insulated and glazed box that has a solar target (“absorber,” usually black) that heats up and then heats the air that is forced through the box by a fan.

If you prefer the practical to the popular, or what you want is heat and cooling, or if PV panels are a bit too rich for your budget, then consider Solar Thermal Systems. If the solar thermal industry was searching for popularity, they may have to rebrand their product: HOT PANELS.

Photo Credits:
“Hollywood:” Kyle Monahan
Heliostat: el biblomatica
Solar flat panel and evacuated tube collectors: Dogstar Solar
Energy use pie chart: energy star
Solar air heat: RREAL
More solar thermal energy facts.