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| 80% solar cells |
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| raptor1956:
--- Quote from: Marco on July 31, 2019, 06:29:47 pm --- --- Quote from: NiHaoMike on July 30, 2019, 01:01:24 am ---Considering that the majority of home energy use is thermal in nature - HVAC and hot water, thermal storage can work very well. What's lacking is incentive to invest. --- End quote --- If you have to carry heat over for a substantial amount of time (months say) you'll need to benefit from cubed laws and use district heating ... and that's a bit pricey. --- End quote --- One potential residential co-generation concept would employ solar cells cooled by water and the waste heat used for domestic heating and hot water. If the cells were 18-20% and you can reclaim waste heat you could up the net efficiency to, perhaps, 25%. You would need some storage for that but it wouldn't be storage over months, just hours. Of course, in a residential solar system the solar cells will usually be locked at a fixed angle so the true net would be perhaps 70% of the max or 13ish% + a few percent for heat. Still, for a typical roof with 100 m^2 of Sun facing roof you'd still be pulling in about 13KW over 4.5-9 hours per days depending on location. As mentioned before the average daily electric usage is about 30KWHrs so such a system would provide all the home needs and also provide daily commute energy for a couple electric cars. Brian |
| Someone:
--- Quote from: raptor1956 on July 31, 2019, 08:39:41 pm ---One potential residential co-generation concept would employ solar cells cooled by water and the waste heat used for domestic heating and hot water. If the cells were 18-20% and you can reclaim waste heat you could up the net efficiency to, perhaps, 25%. You would need some storage for that but it wouldn't be storage over months, just hours. Of course, in a residential solar system the solar cells will usually be locked at a fixed angle so the true net would be perhaps 70% of the max or 13ish% + a few percent for heat. Still, for a typical roof with 100 m^2 of Sun facing roof you'd still be pulling in about 13KW over 4.5-9 hours per days depending on location. As mentioned before the average daily electric usage is about 30KWHrs so such a system would provide all the home needs and also provide daily commute energy for a couple electric cars. --- End quote --- You're out by quite a margin for the energy needed to run multiple cars, average transport use is surprisingly high: http://www.withouthotair.com/c3/page_29.shtml that same publication even has an example of a two person house that went full solar: http://www.withouthotair.com/c6/page_40.shtml 268m2 There are dual function solar+thermal panels available from several manufacturers, but mostly at "low" temperatures so you can't run a heat pump or high temperature storage from the thermal loop. It was a more promising option with concentrator systems but the price of silicon cells is so low that those are a historical footnote now. Very few buildings make good use of their already existing thermal resources so a few well placed fans and/or louvres can be a better investment. |
| NiHaoMike:
--- Quote from: Marco on July 31, 2019, 06:29:47 pm ---If you have to carry heat over for a substantial amount of time (months say) you'll need to benefit from cubed laws and use district heating ... and that's a bit pricey. --- End quote --- A few days of storage will help a lot for leveling the load. More than that goes well into diminishing returns. |
| james_s:
Using water to cool the solar panels and then using that water to heat something relatively low temperature like a swimming pool might be an effective setup although obviously the number of people who can make use of such an arrangement is going to be rather small. It might also work well to heat a water tank that then feeds the warm water into a conventional hot water heater or tankless unit, reducing the need for fuel without needing the solar panels to be as hot as you want the water. |
| Kleinstein:
Cooling the cells with water would only make sense if there is a relatively low temperature heat sink. So something like the swimming pool, or initial warming (to some 20-25 C) of warm water. A heat pump system could be another case, either direct, or possibly a underground loop that takes heat from the ground in winter. The gain in efficiency should not be very high. I have not looked up the numbers, but it should be in the order of 0.3 % per degree (about the Carnot limit for the extra temperature step). Also take into account that claimed efficiencies for solar cells are usually already for low temperature operation. So its more than a nominally 20% cell would give you 20% maybe 21% with cooling instead of some 16 % if they run some 10-20C hotter. Combined heat and PV to get the heat out at a higher temperature would reduce the PV efficiency quite a bit. Given the relatively low costs for the cells, extra effort in cooling is likely not worth it, unless one has a good use for the low temperature heat. Water pipes and anti-freeze measures can be quite a hassle. |
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