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‘You basically have free hot water’: how Cyprus became a world leader in solar heating
(www.theguardian.com)
this post was submitted on 14 Oct 2024
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I would love to see numbers on how they compare against solar panels+ air source heat pump. I guess the solar heater is cheaper, but I would guess photovoltaic+heat pump probably is more efficient all around and it useful for all your home, not just water.
From the point of view of efficiency, solar heaters beat pv. With pv you need to convert the sun's energy into electrical energy, with losses, then convert electrical energy into heat by extracting it from the outside with the heat pump. If you store it in batteries to heat up water later, there's even more losses. Depending on the outside temperature, there's a limit to what the heat pump can do, which an internal electric heater inside the water tank takes over to reach usable shower temperature.
With solar heating the sun heats water, that's it. Not only you can yield energy from the sun more efficiently, this doesn't need complex electronics or semiconductors, so it's also better from an environmental impact point of view.
And it doesn't have to be either/or. In my country, solar heating usually comes as a package deal with pv heating. Some companies are making hybrid pvs. Solar panels become less efficient as they heat up, so a hybrid pv would use coolant to keep them within their most efficient working temperature and extract that heat to be used for space heating, using radiators or floor heating or forced air systems, and/or water heating. Having both just makes sense, since it makes your pv needs (and your upfront investment) smaller and the maintenance on solar heaters is much simpler.
Can second this: direct heating of anything is always going to be more efficient. Also, only ~25% of incident energy on a PV cell is actually captured as electriciy (see here for theoretical backing), and the rest is lost in a lot of ways, but much of it is converted to heat at the PV cell, and if you're capturing that you're using direct heating anyhow.
That is also true for solar heaters when there is no sun. and there are not many places that have climates where heat pumps cant operate these days.
Also air source heat pumps can have a COP of 3.5 so or higher. so even with a lower efficiency from the PV you might get ahead. You also get gains from the high COP on cloudy days where you might need resistance heating on the thermal one
I dont think you can claim any of them is better without doing the math.
The math depends on where you're living, but what would you mathematically compare here, available required roof area and roof load limits?
Because the SWH, if put in a place like Cyprus, consumes next to no electricity. Pumps and electronics, but even that is pretty nil if the tank is on the upper floors. So let's assume a mild sunny climate where the electrical heaters of both the heat pump and the solar heater are off. The heat pump requires power and the water heater doesn't. As for cloudy days, with proper insulation, a SWH can keep the tank piping hot for a few days.
Personally i don't see how a heat pump could beat a SWH in costs and benefits over time. Off the gate it already starts at a disadvantage on environmental impact and upfront costs. When it's sunny, it loses to solar heating, as it's free vs very efficient. It probably regains some ground during inclement weather and very cold winter days. There's also the degradation of the solar panel, which happens at a faster rate than the degradation of the solar heater. So i guess in the end it boils down to how bad the weather gets where you live and yeah, do the math. There's probably a graph of bad weather days with a point where the SWH becomes less attractive than solar powered heat pumps. But for Mediterranean climates, no contest I'd say. You require less area on the roof for a SWH.
And this is only considering available roof area/weight limits, because being honest they're both free sun energy. But you could use the solar power/money/environmental impact elsewhere.