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Old 04-26-16, 08:00 AM   #29
jeff5may
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OK, so I'm going to try to address the issue of space heating/cooling, combining the off-grid requirement of Jake (Memphis), the phase change storage of Kramer, and the advice of Oil Pan.

It should be possible to install enough solar panels and a small and efficient enough heat pump to satisfy cooling demand with the solar power gathered during the day. The most effective way to do this would be to install two heat exchangers indoors: one that feeds directly off the refrigerant loop, and one that feeds off the cold store. Another intermediate brazed-plate exchanger could be installed that fed off refrigerant and exchanged heat with the cold store. Both the refrigerant-to-air (DX) and the refrigerant-to-glycol (BP) should be TXV metered. When the house calls for cooling demand, the air handler would cool the house air quickly and efficiently. When no cooling was being called for, the BPHX would extract heat from the cold store. The outdoor unit would run constantly as long as the cold store was above a certain destination temperature, let's say -5 degC. Once the cold store froze solid, the outdoor unit would not run unless there was cooling demand called for indoors.

At night, the outdoor unit would not run. When cooling was called for, glycol would flow through the second indoor HX then through the cold store. A large enough cold store would stay at or very near 0 degC all night. Due to the reduced heat load, the cold store would only need to be massive (not supermassive). When the sun came up in the morning, the outdoor unit would start charging the cold store for the next night.

In heating mode, the same strategy could be used. When the sun was shining, the outdoor unit could charge heat into the store while no heat was being called for by the house. There are two factors that make this setup less effective for heating, though. For one, the store would not change phase, so the amount of heat it could sequester would be much less. For two, a balance point exists where an alternate heat source would be more economical to run. At the high end, it becomes inefficient to charge the heat store above a certain temperature. At the low end, the outdoor heat exchanger becomes ineffective at gathering heat from chilly outdoor air. Outside of this range, it is wiser to burn wood or natural gas to meet heating demand. During a cloudy cold spell, it might make sense to level the load out by burning backup fuel, diverting the reduced solar power to the battery bank.

Another need that could be satisfied with this type of system that was not mentioned is domestic hot water (DHW). Jake has built a heat pump water heater, and has proven that a small (dehumidifier or refrigerator) amount of capacity can serve his family and a house full of guests without running out of hot water. A small heat exchanger (desuperheater) could be rigged at the compressor outlet that served only the DHW tank. This HX would not rob an excessive amount of heat from the rest of the system, and would self-regulate at a temperature that would not yield scalding hot water.

A major goal of this type of rig would be to not run off of battery power at all. Space heating and cooling is a huge load, and would lay waste to precious battery power that would better be saved for lighting, ventilation, and electrical infrastructure.
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