Revive solar thermal
rhl_
| Posted in Mechanicals on
I recently had my spacepak air to water heat pump installed.
Having this system makes me immediately want solar thermal to offset the electricity.
I know solar thermal is really really dead but can it be revived?
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I think the consensus here has been that photvoltaic (PV) solar has dropped so much in cost that it now makes more sense to install PV systems than it does solar thermal systems. I don't even know if you can still buy new solar thermal collectors anymore. Your best option here might be to run an electric resistance water heater on a PV system, then use that water heater to run a radiant heating system. Some water heaters are actually rated for space heating systems like this. You basically "charge" the water heater with PV electricity when PV power is available, then use the hot water as heat storage, drawing it off as needed. This is a bit simplified, but basically how those systems operate. PV can also offset your home's regular electric use with a grid tied system, making such systems more versatile than solar thermal systems.
Bill
I have an air to water heat pump that serves heating cooling and DHW.
I have 8.6kw solar pv that does not cover my entire roof but gives me 130% of my pre heat pump energy usage.
I'm going to add more solar but since the energy it generates is exclusively to offset the heat pump I wonder if a PVT panel would make sense as it could tie in directly to my existing glycol air to water system.
The problem with solar thermal is that the sun shining doesn't align with when heat is needed. (Which makes sense when you think about it). There just isn't any good method of storing heat.
If you are grid-tied and have net metering, the grid is your storage.
If you like to make stuff and have a wall that receives a lot of winter sun, you could certainly build a large solar air heater and draw your ventilation air in through that. Design it with a bypass for warm weather days. Gary Reysa has some plans for low-cost solar air heaters on his website: https://builditsolar.com/Projects/SpaceHeating/Space_Heating.htm
About the only thing solar thermal might make some sense for is heating a pool. Outside of that, add more PV panels. Even if those panels are driving resistive heaters, it will still be cheaper for the same BTU output as thermal panels plus all the bits to get it to work.
Since you have a heat pump and if you want to take this on as engineering project, you can size a buffer tank that will be filled by the AWHP to shift some excess daytime PV into night time heat. You can control the speed of the pump feeding the buffer tank to adjust heat pump output to match excess solar. You might also be able to hijack the DHW mode to get extra hot water for this without effecting the operation for the rest.
Same as thermal PV, the cost of this means ROI is pretty much never, but would be fun to play with.
This is what Harvest & SANCO2 are doing ...
https://www.youtube.com/watch?v=u8PAkO70UFY
It seems like there are these solar PVT options where the thermal bit cools the panel off. My overall concern would be the ability to get maintenance given how dead the space is. I somewhat have a hard time believing that there isnt some version of this that makes sense.
The problem is that the “version[s] of this” usually aren't economically viable. I think I’ve mentioned on here before that my work involves datacenters, which produce massive amounts of relatively low level heat. I have, for years, tried to find a way to somehow make that heat useful. I joke that we need a towel laundering facility built next door, but in reality the only thing I’ve been able to come up with is to use the waste heat to heat the parking lot so that we don’t have to salt or plow in the winter. I have megawatts of heat to deal with, but it’s too low grade to be very useful, which makes it uneconomical to try to recover, unfortunately.
It’s not that it’s not possible to do what you’re considering, it’s just that there are other ways that are cheaper.
Bill
It makes sense when the availability of solar aligns with your need for heat. Akos in #5 mentioned heating a pool. Or when it doesn't particularly matter when you get the heat, there are lots and lots of solar kilns for drying wood for example.
If you're off grid, or energy is otherwise expensive, it might make sense. Solar hot water might make sense for a summer cabin.
Rhl
A specific concern I'd have in this case is that is that preheating water with solar thermal steals from the efficiency of the awhps, that is if it even works. Some unitary hpwhs, for example, shut down when inlet water temp is too high (say from a preheated buffer tank). Many central water heating systems have their own swing tank on the recirc for this reason, and even if it's a heat pump, it operates at a MUCH lower temperature.
All this to say, stratification in a conventional DHW setup is critical to achieving anything like the rates efficiencies of the awhps unit. If you're able to switch back and forth from a solar thermal system and an awhp system, that might work better. But then economics would come in to play. Its starting to sound like a rube goldberg setup. It's all doable, and could be done cheap DIY, but then you're paying in patience and attention.
When you use solar PV and then use that power through your heat pump, you gain a whole lot of efficiency. And the solar panels are a LOT cheaper.
The cheapest way to do solar thermal would be to buy some PV panels and wire them directly to a DC heating element in a water heater. You can get panels as cheap as $0.20/w these days. Cheaper if you buy used or blemished. Just do not try and use AC relays/switches with DC power.
A solar thermal system is going to be $3-4000 just for equipment. And at the end of the day, stores maybe 10kw worth of energy. You can buy a 14kw battery for that price, which gets you 30+kw of heat out of your heat pump. A battery like that would let you take advantage of low night time rates or store energy from a PV system.
Ryan, a path I'd like to see explored would be using a low end solar thermal panel (like a pool heating one that's highly efficient at low water temps but also extremely cheap) to raise water temp from cold to slightly less cold. That'd then be paired with a water source HP. The benefits would be higher efficiency than PV and ideally costs closer to PV than a high temp solar thermal system. You'd be able to store daytime solar easily to use overnight, maximizing the COP improvements.
Wondering out loud-- what if the best way to improve overall COP might be to do the opposite-- have the heat pump lift the water to say, 100 degrees, and then have the solar thermal (e.g. vacuum tube) do the final lift up to 120 or beyond.
Going in that direction probably gives you less control over the final temperature of the output hot water though. If you preheat the input water to the water heater, then the water heater's thermostat is still in control of the final temperature of the water.
A tempering valve might be a good idea in a setup like this, to have a backup to make sure you don't get excessively hot output hot water regardless of the system configuration.
Bill
A tempering valve is essential!! And a really good one at that.
I used to take my showers from a mon-pressurized $300 vacuum tube setup, and it was venting steam at the end of each day. Manually tempering it was kinda tricky when mixing in with a cold pressurized source. But I learned...
The problem with solar thermal is you have to deal with freezing, venting surplus production and irregular output temperatures. Just dealing with those three things ends up costing more than a PV setup.
The way I would tie a solar thermal into a heat pump heater is with a mixing valve.
For the solar thermal, get a reverse indirect tank (DHW through the coil, solar water in tank). This tank can also serve as your drainback if you don't fill it all the way to the top.
The hot water from the solar goes the cold input of the mix valve. The mix valve will automatically use as much of the solar water as possible, if the solar is hot enough all your hot water will come from solar thermal. If not hot enough it will be used to mix down the output of the HPWH. This gets the most use out of your solar hot water as no matter the temperature, the system will use it.
You will need a 2nd mix valve connected to cold water after this setup to mix down to 120F if the solar is too hot.
Simple drainback solar thermal doesn't have any issues with freezing or overheat.
Hydro Solar has a "hybrid solution" ...
https://hydrosolar.ca/blogs/advanced-technical-zone/how-do-hybrid-pvt-panels-work
excerpt:
"Hybrid PVT panels are a combination of solar PV and solar thermal panels, installed in series. The first two layers are the the transparent insulation layer and of the photvoltaic cells. PV cells produce DC electricial power.
Below the PV cell, Heat accumulator absorbs the remaining solar energy, that is not converted into electrical energy, and transmits this energy to hydronic coil where a heat transfer fluid is heated.
The Heated Fluid can be used for various applications such Space Heating, Process Heating, Domestic Hot Water and Swimming Pool Heating and many other applications."
As to whether Solar hot water is dead...
I'd say that's only a finding in the wealthy burbclaves of the global north, where we have invented ingenious (and colorable) ways of making solar hot water expensive.
In many less affluent parts of the world (i.e. the global south), solar hot water is predominant because its the cheapest, even with occasional frost events. I work with Israeli architects sometimes and they've once expressed astonishment with how expensive solar hot water is in the U.S. (as opposed to Israel, where it's a no brainer, even with peak lows below freezing) With freeze protection, and a lot of other elements that DC pointed out, it wasn't too hard to explain to them why.
Relative to incomes, in much of the world energy is quite expensive. As I noted above, the more expensive other sources of energy are, the more sense solar makes.
As to those "other elements," it's not like they are the result of misguided government mandates or something. They are the direct result of climate and physics.
Back to the OP. Is an automated , expensive freeze protection always necessary? (No). Is it PRUDENT? Perhaps... Thats their decision to make.
Let's assume that they want a cost effective (i.e. cheap) and efficient deployment for solar thermal within the context of his existing air to water heat pump system. I'd argue that there may be and we should help them explore the tradeoffs within that solution space.
Luke,
This discussion reminds me in some ways of the ones we have had around solar thermal for fifty years. There have always been enthusiasts that have made it work with attics full of water carboys, or elaborate mechanical rooms replead with pumps and tanks.
I toured a few of them in the 1970s and 80s. Their owners were rightfully proud, but later struggled to find anyone willing to buy them when they wished to move on. Like a lot of the discussions on GBA, I think it's important to distinguish between solutions that will be palatable to building science enthusiasts, and those that could find acceptance in the larger market-based housing stock.
Couldn't agree more...
This site invites both kinds of people -- thos that are interested in bespoke rube goldberg DIY solutions, and those that want dummy proof and durable.
this is a very pertinent trade off
Malcolm --
There's an important other category, which is systems that never worked, despite an enormous input of time and money by the owner. Particularly houses where had even rudimentary engineering been done before construction, it would have been clear that they were never going to work. Because a lot of those houses dot the landscape too.
As an example, there's the "Sunrise House" in Fairbanks:
https://www.greenbuildingadvisor.com/article/the-sunrise-house-in-fairbanks
I have a neighbor who bought a house in the 1970's with a Trombe Wall (see https://en.wikipedia.org/wiki/Trombe_wall ). After living in it for a while, and thoroughly analyzing the performance, he tore the house down and built a new one, there was no way it was ever going to work.
DC,
Very true, and I'm sure that category is sizable enough it should be acknowledged.