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Tradeoffs between photovoltaics and thermal!solar, as solar power goes.

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Am I to assume the tradeoffs depend on the scale on which they're being used?


For photovoltaics, I'm not sure industrial use would necessarily be any more efficient than household use, on a per-panel level, but for thermal!solar, it sounds almost like the reasons for higher efficiency follow from thermodynamics itself... larger array of mirrors mean more sun rays converged means more sun rays on the same area means higher maximum temperature meaning higher difference between warm and cold reservoirs of the heat engine, which from what I recall from thermodynamics is more efficient, all else held constant. How do the initial costs and maintenance costs of each compare, and how does that depend on the scale on which the photovoltaics and/or thermal solar are being used?

In light of the above, it makes sense that major solar installations go for the array-of-mirrors approach. But what about on the office or household scale? If one were to purchase a half-pipe to line with tinfoil for use as a solar collector, would it eventually pay for itself in terms of what you save on maintenance compared to, let's say, some solar panel? Or would a half-pipe lined with tinfoil cost more in maintenance anyway? What about if one intended to use the halfpipe solar collector not to produce electricity, but to heat water directly for household / office use. (Eg. Tea, coffee, cooking, etc...?) Would that be more efficient than using solar panel electricity for heat?

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On 3/11/2022 at 6:28 PM, ScienceNostalgia101 said:

How do the initial costs and maintenance costs of each compare, and how does that depend on the scale on which the photovoltaics and/or thermal solar are being used?

Hi.  If the equipment installed cost is minimal, everything tends to work well for the pocket.  The marketing of solar water heaters are made for the pockets of vendor and installers to be happier than the customer.   Heating water from photovoltaics does not offer a good efficiency as the photovoltaic panels are only ~18% efficient; and heating panels at least triple that.

Heating water differs a great amount on your location climate.  You may need to shield the elements from wind and cold losses... or not, with a great repercussion in costs.

Last year I made a heater for a friend in Florida and he cannot be happier.   Installed in series before his electric water heater turned very lazy now.

well/municipal water source ------->solar heater------->electric water heater------->house distribution

The method I used very similar to 30+ metres thin wall black 2 inch cheap irrigation polyethylene pipe as :

outdoor shower hot water Archives - Outdoor Shower Company

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At the household level - it is looking more cost effective for us personally to replace our dying solar hot water system with a heat pump hot water system and run it off the rooftop solar. Note, I'm in Eastern Australia with a mild climate and good solar availability. The power used wouldn't deprive us of any, but the amount of electricity exported back to the grid would be reduced, foregoing a small return payment. The upfront costs are similar, with heat pump costs declining but solar hot water systems struggling to achieve further cost reductions.

Why a very simple, direct heating method with no moving parts should be more expensive than a heat pump isn't clear to me but I suspect material costs account for a lot of it - a lot of stainless steel, copper, aluminium and glass in a domestic solar hot water system. Installation of heat pump hot water at ground level is easier too. Durability matters but assumptions that - having no moving parts - direct solar will last longer isn't clear. The repeated heating and cooling of the collectors takes it's toll - it is leaking now at a joint within the collector part and more hassle than it is worth to attempt repair. That part has it's own fluid, originally glycol but now just water, being topped up regularly until we can replace the system.

For electricity grids photovoltaics have significant advantages over solar thermal regardless of energy conversion efficiencies. The panels don't need precise alignments or tracking. Little maintenance is needed. They are lower cost per watt hour compared to high temperature Solar Thermal - enough lower that it is cheaper to add more area of panels than use tracking. Finding room for more isn't an issue.

Solar thermal's mirrors are technically demanding - precise surfaces and precise solar tracking for each mirror are essential. Steam turbine efficiency in general is rarely above 50% - more like around 30% - but I'm not sure what they are for working solar thermal plants.

The capability to store energy as heat - molten salt usually - should be one of the significant advantages but I think including it has actually made the economics worse, not better; attempting to be some kind of drop in replacement for 'baseload' fossil fuel plants was a mistake I think. It added costs but without a clear market and grid demand for stored energy the grid managers just call on lower cost power from elsewhere and the storage component of these plants fail to earn money. Photovoltaics pass off the load leveling role to other elements of an electricity grid, which is normal foer how grid managers deal with the ups and downs of supply and demand. Calling on power from somewhere else has, so far, been cheaper and easier than each generator providing it on site. The economic of solar thermal storage may change as the value of stored energy becomes more explicit within electricity markets.

Edited by Ken Fabian
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