In the previous blog I discussed the concept of passive solar architectural design and the potential for system autonomy. On site power can come from different renewable sources like a solar, wind or fuel cell powered electric generating system that provides annual net-zero energy consumption to completely power the house. In California there are many well established solar electric systems providers and the financial incentives (tax rebates and low interest loans) that help make it feasible to have free electricity. There are on-grid and off-grid solar electric systems; the on-grid essentially uses the commercial power grid as a battery and back-up. The idea of free electric power is exciting and quite feasible in many climate zones, See graphic, below.
(To see a larger version of this image, click here.)
The automated control devices are: differential temperature sensors, microprocessors and timers, systems designed by an energy engineer or air conditioning contractor, to turn on the heat pump system when heating or cooling is needed. These are standard off-the –shelf devices that are readily available, dependable and of modest cost. Differential thermostats monitor temperatures at various locations, say inside and outside air temperatures. When the inside temperature falls low on the comfort scale setting and outside temperature or the ground temperature is warmer, the thermostat will tell the heat pump to turn on and deliver heat where needed. The heat pump can be a ground-source (geo-thermal) or an ambient-outdoor-air-source system to provide the auxiliary heating and cooling required for my passive solar house. The passive solar design itself will vary considerably from microclimate to microclimate, but that is another story to be discussed at another time. The heat pump is a machine that runs on electricity and uses a phase change heat exchange system to provide heating or cooling as seasonally required. By “pumping heat” the exchanger either, extracts heat from, or dumps heat to, a large thermal mass like the ground or the atmosphere. The “ground source” might be a water well(s), buried tubes in the ground (one contractor is placing the heat exchange tubing in the bottom of the septic system leach field) or even a body of water like a pond or lake. The “air source” is generally the air around your house and it is not as steady a temperature as the ground on a day to night (diurnal) basis.
The pumping “medium” is a transfer fluid (gas or liquid) that takes on or releases heat to the earth for the system to provide heat or cooling as needed. When the interior system thermostats and controls call for heat the heat pump extracts heat from the earth’s crust (or air); when cool air is called for it pumps heat from inside the house into the ground(or air) either dumping heat or extracting “coolth”, See graphic, below.
(To see a larger version of this image, click here.)
Ground-source heat pump systems are more effective than outside-air heat pump systems because the earths mass has a dense thermal heat sink in which to store, or extract, thermal energy. Air-source systems are better for cooling, and in cold climates they require another source of heat like gas or electricity to supplement the heat they can extract from the air; they do not work well below 40 degrees F. However air systems are much less expensive to install and they can work well in moderate climates that do not experience widely fluctuating daily or seasonal temperatures. When you combine an independent energy source like solar electric system with a heat pump system you are creating the basis of a perpetual motion machine. To be continued….
