The Self-Sufficiency Handbook. Alan Bridgewater

the walls, floors, and furniture. If the night is warm, we throw open the windows to let the heat out.

      If we take the direct-gain idea one step further, we could create thicker walls and floors, with more masonry and concrete; use more insulation in the floor, wall, and ceiling cavities; have larger areas of glass that are positioned so the sun’s rays will strike them at right angles; and use darker colors inside the house, thus helping radically increase a house’s capacity for heat storage.

      Whereas the passive, direct-gain approach is absolutely fine in a temperate climate where the days and nights are mild, it fails in a hot-cold climate where the days are hot and the nights are cold, because the rooms will be uninhabitable at midday.

      Passive Indirect Solar Gain and Trombe Walls

      With indirect solar gain, a heat-absorbing wall of black-painted masonry or concrete, or a tank of fluid, is set between the window and the interior so that it receives full sunlight. With this system, known as a Trombe wall, the sun heats the wall, and vents in the top and bottom of both the window and the wall are opened and closed to utilize the stored heat. The sun shines through the glazing and heats the masonry wall, with the effect that the space between the glazing and the wall becomes a thermal chimney. Then, the vents that are set at floor and ceiling levels in both the glass and Trombe walls are managed so that the currents of hot air that rise by convection between the wall and the glass—in the thermal chimney—are directed either in or out of the building.

      Solar Collectors

      When we first became interested in self-sufficiency in the 1970s, people were experimenting with different types of homemade heat collectors built from old central heating radiators, black plastic tubes, rubber inner tubes, and so on. Commercial systems were coming onto the market at that time, but they were generally bulky and very expensive, considered alternative and hippie, and, for the most part, not very reliable.

      One system I remember was amazingly simple: water trickled down over a glass-covered galvanized steel roof into a trough and then into a tank in the cellar, where it was used for hot water and space heating. This system did not look very pretty, and it was a huge free-standing structure almost as big as the side of the house, but it was amazingly efficient, with cold water going in at the top end and too-hot-to-touch water coming out the other end. There was another system in which black plastic pipe was wound around and around a massive cylindrical house-high storage tank. Just as with the previously mentioned system, cold water went in at one end and came out hot at the other.

      These days, commercially built solar collectors are not only available at very reasonable prices, but they are efficient, sophisticated, and generally well-designed devices. If you simply want to cut energy costs and are looking for a tried-and-true method, solar collectors are good options.

      Trombe Controls

      Hot day/hot night: In daytime, the vents in the window are open, and the vents in the wall are closed. The air in the space rises by convection; hot air passes out of the two vents at the top of the window, drawing cool air into the bottom vent. The circulating air helps cool the interior.

      Hot day/very cold night: In daytime, the top vent in the window and the bottom vent in the wall are open, and the other vents are closed. An additional window/vent toward the back of the interior is open. The air in the space rises by convection; hot air passes out of the top window vent and draws cool air through the interior space.

      Hot day/cold night: In daytime, the vents in the window are open, and the vents in the wall are closed. The air in the space rises by convection; hot air passes out of the two vents at the top of the window, drawing cool air into the bottom vent. At night, the vents in the window are closed, and the vents in the wall are open. The hot air in the space between the wall and the window rises and passes into the interior.

      Cold day/cold night: In daytime, the vents in the window are closed and the vents in the wall are open. The air in the space rises by convection; warm air passes into the vent at the top of the wall and heats the interior. At night, all vents are closed.

       How Does a Solar Collector Work?

      Although there are various compact and non-pump solar heating systems, most solar-collecting setups use one or more pumps to push the water/antifreeze/oil around the system. Solar heating systems usually comprise five primary elements: a collector, a hot water storage cylinder, a panel of controls and sensors, one or more pumps, and pipework. This is how it works:

      

      One type of solar water-heating system.

      • The sun heats the heat absorber in the collector.

      • The heat from the absorber is transferred to the water/fluid.

      • The controls switch on and set the pump(s) in action at a designated temperature.

      • The hot water/fluid is pumped from the collector to a loop/heat exchanger in a storage tank.

      • The water from the storage tank is used either directly as hot water or for space heating.

       Types of Collector

      There are three primary types of collector, all with slightly different qualities and characteristics. Research the options until you have a clear understanding of which will best suit your needs.

      Flat collector: The collector is made up of radiator-like channels/tubes/pipes that sit on a thin, heat-absorbing sheet within an insulated box and glazed box—very much like the old 1970s prototypes. The water/fluid in the tubes draws heat from the absorber.

      Wide-angle concentrator collector: The collector is a copper tube complete with fins that sits within a shaped collector housing. In many ways, this design is better than the flat collector because the curved shape focuses the sun’s rays on both the front and rear surfaces of the absorber plate.

      Evacuated tube collector: The collector is a series of transparent glass tubes that have contained within their bodies an inner and outer tube, a vacuum, a heat-absorbing surface, a mirrored heat-reflecting surface, and a copper heating pipe. The sun’s heat is absorbed by a coating on the inner glass surface; the heat passes to the tip of the heating pipe and in turn is transferred to a copper manifold and then on to the storage tank.

      Photovoltaic Cells

      In the late 1970s, photovoltaic (PV) cells—a spin-off from the space race and its associated industries—were developed as a means of directly converting light into electric power. No water—just sunlight that directly became electricity. At that time, these cells were so incredibly expensive that people thought of them as being beyond domestic use. Now, of course, it’s common to see PV panels in places where they provide small amounts of off-grid power. Mass production has resulted in much lower prices than when they were first introduced in the ’70s.

       How Do Photovoltaic Cells Work?

      A PV system converts sunlight directly to direct current (DC) electricity. A typical PV cell structure—made up in layers—has a back contact, two silicon layers, an anti-reflecting coating, a contact grid, and an encapsulating surface. As to how it works, it is enough to know that when the structure is bombarded by photons from the rays of the sun, a steady flow of electrons produces a minute amount of electricity in the form of a DC. Certainly, the amount that each cell produces is small, but if you connect a series of cells in the form of a panel, and use an inverter to turn DC to alternating current (AC), then you have a relatively simple and inexpensive energy source.

      Solar Collectors