A low-cost concentrating solar building design with triple the efficiency of PV panels could solve long-standing challenges in the building-integrated solar sector. It would furnish heat, air conditioning, and hot water, which together comprise most of a typical building's energy needs.
A concentrating solar building as a thermal system is highly efficient in its use of the entire spectrum of solar energy, whereas photovoltaic panels utilize only a narrow band of visible light. These buildings, however, would best be deployed in sunny locations and should be oriented along east-west lines. This way, the receiver can track seasonal variation in the sun’s position. That said, a concentrating solar building would have few moving parts, none of which are mounted on the roof, which would simplify maintenance. Mirror cleaning would be as simple as pressure washing your gutters.
At the heart of the technology is a gently curved mirrored roof, arguably the most efficient way to capture abundant solar energy cheaply in a building. A cylindrical trough shape for the mirror produces consistent energy despite changes in seasonal solar angle. A thermal receiver located above the mirror tracks moving focus, while a safe antifreeze solution carries heat from the receiver into the building. This thermal energy supplies space heating and hot water but also cools the building through an absorption chilling process that utilizes safe lithium salts.
Absorption cooling, around since the 1800s, uses heat to drive chemicals in and out of solution to effect cooling. The most efficient and enviro-friendly techniques use lithium salts in dual or triple-effect processes that require high temperatures. This is beyond the thermal capability of simple, one-sun hot water panels but exactly where solar concentrating systems excel.
You might have seen the long parabolic trough mirrors used in CSP (concentrating solar power) systems. They are very high in concentration, but with a major caveat: parabolic mirrors must track the sun precisely to maintain concentration. Cylindrical mirrors, on the other hand, provide less concentration but collect solar energy consistently from a fixed position. Reflected focus of these mirrors moves around seasonally, which can be tracked effectively with relatively small linear receivers made up of the same evacuated tube collectors used in CSP systems. What’s more, shallow cylindrical mirrors can be easily formed from roofing sheet metal and mirror film. Simple in shape, cylindrical mirrors would integrate readily into building roofs for cost-shared savings.
For the technically inclined, cylindrical mirrors belong to a class of optics called non-imaging optics. When used as primary mirrors in concentrating solar energy systems, these mirrors typically have long axes of curvatures oriented east-west. A linear receiver, also aligned east-west, tracks across the mirror’s width as solar declination varies seasonally. The annual extent of solar declination is approximately 47 degrees at latitudes between 30 and 40 degrees where promising solar sites abound. This relatively narrow declination range means the linear receiver need only move a tiny increment each day in order to collect solar energy.
How does a concentrating solar building actually deliver the energy goods? A radiant floor system is a promising delivery system for heating and cooling. Radiant floors are expensive retrofits, but we’re talking new construction here, so installation costs would be more competitive with forced-air systems. Storage tanks for night-time heating and cooling would be located in the building’s side bays.
ClimateWell, an innovative company in Sweden (www.climatewell.com) offers a solar heating and cooling system that goes a step further with its use of chemical storage and smaller tanks. A concentrating roof system connected to ClimateWell’s storage system and a radiant floor could prove the ultimate ticket to effective solar heating and cooling of new buildings.
The receiver arm over a concentrating solar building will be questioned by some, but buildings throughout history have sported all manner of turrets, flags, chimneys, and antennae. If we extrapolate into the future, a solar receiver arm above one’s house could be construed as evidence of progress in energy innovation.
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