Photovoltaic-powered heat pumps?

[SimpleSunnyPool]

We live near San Jose CA. The air is warm Apr to Oct, but our water gets cool mid-Sep to mid-May. We’re upgrading the never-replastered 40-year-old pool with an auto safety cover, variable speed pump, etc. It’s 20k gal 12’x36’, and I’d like it to be 80°F Apr-Oct. A gas heater costs ~$1500, and Rheem estimates gas would cost $400/yr. A heat pump costs ~$3000, and Rheem estimates 1500 kWh (Hayward 2000 kWh), though 150 kWh (300 kWh) is Jun-Aug when the solar cover alone has sufficed, so perhaps that’s pessimistic. CA power is tiered, so my first kWh costs $0.13 and my last costs $0.34. Installed solar thermal ranges from $5k-$8k.

Gas seems cheapest, but costs will only rise, it seems wasteful, and I fear we won’t use it. Solar thermal is expensive upfront, requires perhaps 1 kW more pump power, and runs water over my roof. We could offset a heat pump’s electric usage with solar photovoltaic panels (PV): at 1.75 kWh per year for each watt at my location, and $4/W installed ($2/W materials), running a heat pump off grid-tied PV panels is about the same cost as solar thermal. Versus gas, it’s like a 20-year bond that yields 5.5%, which is a decent return. Unlike solar thermal, when the heat pump is not used the PV generates useful power, and the heat pump can generate heat even if it’s cloudy. PV also generates perhaps 6x more useful BTUs/sqft, which gives us room to offset more of that expensive $0.34/kWh electricity. The bad: a heat pump is loud, upfront costs are high, and electric use will be high in Apr/Oct.

I know this has been a controversial topic in the past, but PV is cheaper now, I only need heat a few months of the year, and I live in a sunny climate. So, what do you think?

 

[chem geek]

I don't know where you get that "PV generates perhaps 6x more useful BTUs/sqft". Solar thermal panels are roughly 80% efficient while PV is roughly 15% though depends on the specific panels you get. Even using a heat pump, you'd have to have a Coefficient of Performance (COP) of around 5 for PV to be equivalent in net heating efficiency to solar thermal. In practice, your COP will likely be lower.

Nevertheless, you are correct that with PV you can at least use the sun's energy for electricity usage even when you aren't needing to heat the pool.

As for gas, I think their estimate of $400 per year is low though it depends on how far into April and October you want to heat the pool. Look at this post for our actual costs for gas heating our 16,000 gallon pool, but note that we are heating to a warmer 88ºF compared to your 80ºF and that makes a big difference. Nevertheless, you should be aware that an automatic safety cover only (that you and I both have) insulates about half as much as a bubble-type cover. So whereas a pool with no cover might lose 6ºF overnight, a safety cover cuts this to 3ºF but a bubble-type cover might cut it to 1.5ºF.

Your average day/night temperatures (see below) are a little warmer than mine, but even so you aren't going to get to 80ºF with a mostly opaque cover -- you'd need a mostly transparent bubble-type cover to have a gain in temperature and even then it's only around 5-10ºF.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
.58..62..66...69..74..79...82..82..80...74..64..58 ... Avg. High Temp
.42..45..47...49..52..56...58..58..57...53..46..42 ... Avg. Low Temp
.50..54..57...59..63..68...70..70..69...64..56..50 ... Average Temp (slightly more weight to daytime temp)

How were you expecting to get your pool warm enough in the summer? Your solar cover might have gotten your pool temp up to 10ºF higher so close to or around 80ºF, but your auto-cover isn't going to do that. It's not just that it's thinner so doesn't insulate as well, but more importantly it doesn't let any significant amount of sun into the water to heat it. If you get a dark cover, you will get some amount of heating but only a few degrees.

 

[SimpleSunnyPool]

Thank you so much! I think I’ll need a baseline of 1-2 therms/day Jun-Sep, then an additional 2-3 in May/Oct. I missed that baseline load.


Questions
===
* How much heat will I need in the summer? (my estimate: 1-2 therms/day.)
* How much heat will I need the first and last month of a 7-month 80°F season? (my estimate: 3-5 therms/day).
* How insulating is an automatic safety cover? What percent of sun does it trap as heat (my guess: 20-25%)?
* How much additional energy will the pump use when solar thermal is running? (my guess: 1 kW.)
* What COP should I expect from a heat pump in a dry climate with 60°F ambient? 5? 4? 3?


Options: this needs more research
===
* Thermal + PV + heat pump may make sense, but some costs double (both plumbers & electricians — and roofers; both higher head loss and higher $/BTU; …), and we have limited roof space.
* Lots of thermal makes sense if we don’t get PV. Gas plus some thermal may make sense, too.
* $1-$2/W for a PV kit is similar per pool BTU to $6/sqft solar thermal (e.g., wholesalesolar.com or solardirect.com). That doesn’t include the heat pump or installation, which seems to be the major expense.


Numbers
===
Assuming an 80% efficient gas heater, in Oct chemgeek gains 6.4 therms from gas and 3.5 therms from solar, which would raise 16k gal 7.5°F. If 75% of losses are through the cover, the cover must have r-value = 0.4 = (88-64)*(16*32)*24*1e-5 / (10*75%) = temp_diff * area * hr/day * btu/therm * losses. Plugging in temp and r-value, I’d need 7/5.5 therms in Apr/Oct, and 3.5-4 therms in Jun-Sep. If a dark cover captures 20-25% of sun as heat, it will provide 2-3 of those therms. So I’ll need to supplement 1-2 therms/day even Jun-Sep.

By efficiency per square foot, I meant that 15-20% efficient PV panels provide useful energy for a 4-6x cop heater, versus 80% efficient solar thermal that generates useful energy only 25% of the year. 60-120% PV + heat pump / 20% solar thermal => 3-6x more useful energy per square foot. From SRCC data, thermal panels are closer to 60% efficient when the water is warmer than the air.


Notes from last summer
===
I used a lightly-tinted bubble-cover last summer; the auto cover is a planned safety upgrade (this may explain some of my confusion). Here are temperatures I measured last summer after I started to use the cover:
4-21 4-26 5-1 5-11 5-19 … 7-7 8-18 9-7 9-17
65°F 72°F 76°F 78 80°F ... 86°F 83°F 80 77°F
We didn’t even try to swim in Apr or late Sep. We use our pool with kids and for exercise, so its better warmer in the summer (82°F+) but 80°F is fine for exercise.

 

[chem geek]

* How much heat will I need in the summer? (my estimate: 1-2 therms/day.)

Heating 20,000 gallons (166,600 pounds of water) by 1ºF requires 166,600 BTU or 1.666 Therms. I wonder if you are in a warmer part of San Jose than the average numbers I gave because a standard solar mostly transparent bubble-type solar cover isn't usually going to give you a 16ºF gain over average temps that you saw in July. At any rate, if you do need supplemental heating due to a mostly opaque safety cover, it's likely going to be on the higher end of 1-2 Therms per day which is only 1.7 to 3.4º of heating. I typically lose 2-3ºF overnight during the peak summer so you should figure on being closer to 2 Therms/day needed.

* How much heat will I need the first and last month of a 7-month 80°F season? (my estimate: 3-5 therms/day).

3-5 Therms is 5.0-8.3ºF and in May and September with a mostly opaque safety cover this may be about right.

Don't forget that you not only need heat for a temperature rise, but also just to maintain the temperature since there is heat loss so long as there is a temperature difference. I have a gas heater that outputs 2.5 Therms/hour and it is barely able to keep up when there is a temperature difference of around 43ºF (from 45ºF to 88ºF) so that's the amount of heat loss of roughly 1.5ºF per hour though not just through the safety cover but also through the walls and floor and plumbing. You are looking at a roughly 25ºF temperature difference (from 55ºF to 80ºF), but that's still a substantial amount of heat required just to maintain the temperature. Even so, I figure I'd lose around 6ºF overnight in the May and October months (the 43ºF is not normal -- on such nights the pool would lose more than 6ºF) but also lose during the day as well. So your estimate is probably reasonable.

* How insulating is an automatic safety cover? What percent of sun does it trap as heat (my guess: 20-25%)?

The issue is not about trapping heat (i.e. insulating) as much as it's about how much of the sun's energy does it transfer to the pool. As I pointed out, a safety cover cuts heat loss roughly in half and that's almost all due to eliminating evaporation. The heat loss from conduction is barely cut at all because the cover is so thin. If you feel the cover it will feel the same temperature as the water. A bubble-type cover will feel the same temperature as the air and it roughly cuts conduction losses in half (in addition to the evaporation losses, of course). The issue is that it being mostly opaque, it blocks most sunlight. With a mostly transparent cover, sunlight enters the pool and heats it. With an opaque cover, if it's light, it reflects a lot of the sun's energy. If it's dark, it absorbs it and gets hot, but only some gets conducted to the water while the rest gets conducted and radiated back into the air. If you point your returns so that they circulate the water under the cover, that will help.

* How much additional energy will the pump use when solar thermal is running? (my guess: 1 kW.)

This depends on how many panels you use and the size of plumbing to/from the panels and on the usual speed you use when solar is not running. For my pool, it's 1500 Watts when the solar is on at 48 GPM with 12 panels (4 GPM per panel) and 275 Watts with no solar at 26 GPM. So it's 1225 Watts more for me, but I've got very long runs to/from the solar using 2" plumbing (2.5" or 3" would have been better) and the solar is spread across a rather long roof.

* What COP should I expect from a heat pump in a dry climate with 60°F ambient? 5? 4? 3?

You will likely have more than a 20ºF temperature lift (from 60ºF to 80ºF) because you will likely be needing to heat your pool at night if you want the water to be warm in the earlier part of the day -- otherwise you'll be limited to swimming in the late afternoon after a day of heating up to your desired temperature. Nevertheless, your temperature lift isn't huge and according to the graph for a "conventional space conditioning unit" with Carnot efficiency of 0.3, the COP for a 20ºC (36ºF) is 5 so if that's true then you should be at least at 5 even at night. I've never heard of anyone report heat pump efficiencies that high consistently for heating their pool, but we don't get too many such reports.

 

[SimpleSunnyPool]

Here are best-case costs for PV versus typical costs for solar thermal. Solar thermal is still the cheapest option to provide baseline heat.

* Current PV costs per watt: $0.85-$1.3 for panels, $0.5-$0.7 for inverter, $0.25 for mount, $0.3 for misc = $1.90W - $2.55/W not counting installation. Using 290W panels, that’s about $450/m^2 and 1.2 kWh/day per m^2. Assuming COP=5, that’s $75 per kWh/day. If you can arbitrage TOU pricing and sell daytime power at 3x night rates, that’s still $25 per kWh/day. If you only need heat for a few weeks and can offset other usage at a good rate, you’ll get perhaps a 4x better return on investment (e.g., 3 months of heat and 9 months of electricity). Such seasonality could lower the cost to $6 per kWh/day.

* Solar thermal: $250 for a 3 m^2 Gull Sun Coil (we have a flat foam roof). That’s $85/m^2 and 4 kWh/day per m^2 (assuming 80% efficient), or $21 per kWh/day. chemgeek’s 1225W to push water through 472ft^2 of panels is 28 W/m^2, which requires $45/m^2 in solar panels, increasing costs to $32 per kWh/day.

Due to COP, PV delivers more energy per area. If you can take advantage of arbitrage and seasonality, PV may even be cheaper. PV has tax incentives until 2016. You can run a heat pump when you want or profit when you don’t, and PV will continue to get cheaper and multijunction panels will be efficient. Though using a single installer may be cheaper than two, I’m guessing PV installation costs more (potentially much more). PV requires a $3000 heat pump. Which do you prefer, nailing water to your roof, or running a lot of current? (I prefer neither.)

For my use case, I need a baseline of 2 therms/day = 58 kWh/day in the summer, but 3-5 therms/day = 87-145 kWh/day in Apr and Oct. Barring surprising installation costs, I should get at least 5 Gull Sun Coils for $1250. I would need another 2-7 panels for Apr and Oct for $500-$1750. If I otherwise need a heat pump, additional equivalent PV will cost perhaps $600-$2000 assuming a little seasonality and arbitrage. I think we'll get as many solar thermal panels as we have room, and we'll get PV panels to offset non-pool usage to the degree we can afford PV.

> a standard … bubble-type solar cover isn't usually going to give you a 16ºF gain over average temps that you saw in July.

I’m in Sunnyvale, and last July was particularly warm, so the gain was closer to 10ºF.