SOLAR ENERGY 2 – SOLAR HEATING
In my last column we looked at photovoltaics, the generation of electricity using sunlight. This week we will look at solar heating, SH.
SH is something we are all familiar with… remember those black vinyl car seats on a summer day when we were kids… ouch! Let’s see what can be done with this energy.
Solar heaters are available today in several forms, and a reasonably efficient version can be made with readily available materials. If you want to heat your swimming pool, simply putting a coil of black pipe on the south-facing garage roof, connected to the recirculating line will make a difference to your pool, and you can upgrade to more efficient designs fairly easily. In many tropical and sub-tropical countries, rooftop solar heaters are commonly used to supply domestic hot water, with small electric or fuel-fired in-line heaters to make up for cloudy days or night-time use. Passive solar heating consists of laying a floor of stone or tile in a south-facing room and allowing the low winter sun to warm it. Because stone and tile absorb heat slowly and release it slowly, this design can make a small difference to heating a room, even in Canadian winters.
More sophisticated systems can be fitted to homes to supply the bulk of the heating needs, even in Canada. These consist of a non-freezing fluid that is pumped through a flat panel collector on your roof and then through tubing incorporated into the floor, for example. The angle of the panel is critical to obtain efficient use of the sunlight, which is why you may see these mounted at awkward-looking angles. These systems require back-up power, but clearly would reduce the need for conventionally-generated power.
The large-scale generation of electricity through SH is also well advanced. In Spain a system has been in operation for several years. It involves a roughly circular field of flat mirrors, which move to track the sun and reflect the sunlight to a central tower. Inside the tower is a receiver in which a fluid (water) is heated to over 500 deg. C. and the resultant steam turns a turbine to generate 11 MW of electricity. This is over double the output of the photovoltaic plant in Arizona we looked at in the last column. As with photovoltaics, there are no emissions associated with this, but also it takes some land area, and more maintenance is required. Not everyone wants a 300 ft. tower in their neighbourhood.
The latest designs involve parabolic reflector technology. A parabola is a curve, a bit like the narrow end of an egg, which has the very useful property of focussing all the incident light onto one spot. So a parabolic reflector, in the form of a trough with a pipe containing water or glycol solution (similar to the fluid in your car radiator) running along inside it is a very effective way to capture solar heat. The catch (there is always a catch) is that the reflector has to track to sun to maximise efficiency. The good part is that you don’t need a tower, but you pipe the heated fluid directly to a generator building. Again, there are no emissions but maintenance is required for pumps, generators, and tracking mechanisms, as well as ensuring that the mirrors are kept clean. As with PV systems, there is a need for some form of storage so that energy can be generated at night and in cloudy weather. With SH systems, the latest technology involves heating a salt solution and storing it in a tank. The stored heat can be used to generate steam to run turbines and generators at night.
The world has about 500 MW of installed capacity, about 100 MW of planned capacity and about 2700 MW of announced intent to build capacity. This is a small but significant portion of our needs. All of this is in tropical or sub-tropical locations, and the newest plants use the parabolic trough technology, with computer-controlled tracking to obtain the highest efficiency. Efficiencies of about 40% have been claimed, compared with 15% for PV systems, based on the conversion of sunlight to electricity.
Currently the largest operational system is in California’s Mojave desert, with 354 MW capacity. This could supply over 300,000 homes. Even at this scale, the cost of SEGS electricity (solar energy generating system) is three to five times the cost of conventionally generated electricity. However, as we are painfully aware, the cost of fuel (natural gas and oil) is certainly not going down, so the gap is likely to narrow quickly.
So you can see that both photovoltaics and solar heating can play a significant part in meeting our energy needs, even with their known drawbacks. Their key advantage, that of generating power without accompanying greenhouse gas emissions, can only help in ensuring their widening adoption in the coming years. As well, if governments, utilities, and regulators are serious about reducing greenhouse gas emissions, they will have to encourage this kind of development through tax incentives, price supports and other inducements. Only through these actions will we see a meaningful advance in solar technologies of all types.
Rod Charlton was born in England and came to Canada in the early '50s. He worked at DuPont Research Centre in Kingston for 10 years, and at DuPont Maitland for 25 years, retiring in 2004. As a science educator, he has presented "Fun with Science" about 100 times at local schools and has been involved with Rideau-St. Lawrence Science Fair for over 25 years. He is married to Bonnie Charlton and has three grown daughters and three grandchildren who like science. Rod can get by in French if he has to, and enjoys highly-amateur golf. His science column appears every second Saturday in Brockville, Ontario's paper The Recorder & Times.