Hot water options

techman · Oct 22, 2014

Despite the complexity of thermal panels discussed, water heating is many times more efficient than PV to an electric or heat pump water heater. The newest systems have a lot of smarts in them, and many limitations and problems mentioned are rare on newer systems. Sure you need heat exchangers and more piping but a couple of panels will make most of a families hot water. Compared to PV, a roof full of panes would be needed to make the same hot water. PV does have the ability to supply useful power when the HW is at temp, which may be an advantage, but for just HW thermal panels are the logical choice.

I have PV only, with ground source heap pumps, and my utility bills are quite reasonable. I considered adding a couple of water heating thermal panels, but I do not have enough clear sight space on my roof to be effective.

Paul

TnAndy · Oct 22, 2014

quicksandfarmer said:
With advances in photovoltaic efficiency and the development of heat-pump water heaters, photovoltaic systems are now quite competitive for heating water.

Here is my effort to do an apples-to-apples comparison of the installed cost of a solar thermal water heater, versus the installed cost of a heat-exchanger electric water heater and a solar electric installation large enough to support it. A lot of the specifics of the comparison depends on your assumptions, so Iå€¤l start out with the assumption that you need 40 gallons of hot water a day, and the water has to be raised 60F degrees. So thatç—´ 19,920 BTU per day, or 5838 Watt-hours per day.

Iå€¤l do the electric first. Over the course of the year, where I live averages 3980 Watt-hours of insolation energy per square meter. Commercially available panels (see Grid tie Solar Power Systems - Grid tie Solar Panel Systems) have an efficiency of 14.4%. Assuming a heat pump efficiency of 200%, to get 5838 Watt-hours of heating you need 2919 Watt-hours of electricity. At 14.4% efficiency, to get 2919 Wh of electricity you need 20,271 Wh of insolation. At 3980 Wh/sm you need just about exactly five square meters of panels. Those commercial panels are 1.6 square meters so you would need three of them. These panels are called 250 watt panels so this would be a 750 watt system. A complete 750 watt grid tied system kit costs about $1200 (See 1200W 900W 750W 600W 450W 300W Complete Kit Grid Tie 160W Solar Panel System | eBay, I'm not recommending it, it's just the first one I found on Ebay) . Youå£‡ also need a heat pump water heater, a GE Geospring 50 gallon is about $900. So $2100 for the pieces. IçŸ¥ going to assume that the installation cost is going to be roughly the same for either system since the pieces are fundamentally similar. IçŸ¥ also leaving out subsidies and credits because they seem to be similar as well.

Now for the thermal system. Interestingly, evacuated tube panels have almost completely replaced flat panels in the marketplace, I wasnç¨š even able to price a flat panel kit any more. Evacuated tubes have come way down in price and offer superior cold weather efficiency. A thermal system has to be sized to provide 100% of the energy needed on the lowest-output day of the year. Here, December is the lowest insolation month of the year, averaging 1810 watt-hours/square meter per day. The efficiency of thermal collectors depends on the temperature. The average daytime temperature in December here is 47F, a commercially available collector gives an efficiency of 27% at 47F (see Solar Water Heaters with Vacuum Tubes ) Efficency is lower in January when the temperature is lower, but January is sunnier, so December is the lowest producing month. To produce 5838 Watt-hours per day with 1810 watt-hours/square meter of insolation and 27% efficiency you need 11.9 square meters of panels. Those commercially available panels come in arrays of 18 tubes, each one is 2.35 square meters, so youå£‡ need 90 tubes. They donç¨š come that big off the shelf, but you could get a 72 tube system for $4675 and an additional 18 tubes for $500. So $5175 for the pieces.

So $2100 for the PV and $5172 for the thermal, itç—´ not looking good for thermal. Now thermal systems are often sized to provide less than 100% of the hot water needs, the costs are much better that way. Often they are sized so that they provide 100% on the best day of the year instead of the worst day, that way 100% of the energy they provide is utilized. The best month for solar here is August, if you sized your system to provide 20,000 BTU on a typical August day it would only have to be 37% of the size. So instead of 90 tubes you could have 33 tubes. A 30-tube system is $2450, 36 tubes is about $125 more. Such a system would provide an average of 14,500 BTU per day year-round, or 72.5% of your annual heating need. Itç—´ still not cost competitive, but itç—´ much closer, within a few hundred dollars. However, you would need a backup heating source to provide that other 27.5% of your hot water.

The advantages of PV swing even further if you look at two other factors. The first is economy of scale. The bigger your system, the less it costs per installed watt. That 750 watt system is $1.60 per installed watt. However, if youæ±*e already installing a PV system the marginal cost of three additional panels is the cost of the panels. Those 250 watt panels cost $210 each at quantity 25, so thatç—´ $0.84 per installed watt. The second factor is space on the rooftop. If there is limited space, PV makes even more sense. The 90-tube thermal system occupies 128 square feet. Even the modified 36-tube system occupies 47 square feet. The three-panel 750 PV system occupies 50 square feet. So if you only have limited space PV is a better use of that space.

It varies by state, but here the electric company allows reverse metering, so excess electricity produced by the PV panels is returned to the grid and I get a credit on my electric bill. Excess hot water produced by a hot water solar panel is just lost.

Where the jury is still out is on the efficiency claims of the heat pump. Interestingly, even without the efficiency of the heat pump PV is still more competitive than thermal. Whatç—´ telling is IçŸ¥ starting to see gadgets that allow you to connect your PV panels directly to a resistance-heated water heater, bypassing the inverter and the grid-tie when the panels are producing and hot water is needed. See Solar Hybrid Hot Water Solution - No Plumbing Mods Required - At TechLuck

Without going thru all your numbers, I can tell you right off the bat a 750w, $1200 PV system WILL NOT produce 5.8kwhrs per day. You are simply WAY off in your numbers there. I didn't wade thru the rest of them.

Assuming the panels produced a full 750w (they don't) for a full 6 hours of sun (more like 2, the morning and afternoon hours will fall off away from 11am to 1pm peak production), you'd only get 6 x 750, which is 4.5kwhrs....and in real world experience, you'd find more like 2.5-3.

Here is a 10,000w system I just put on line for a guy in Tennessee. First month of production was 1240kwhrs. Had that been a 750w system, it would have done (750/10,000 x 1240) 93kwhrs in that month......slightly over 3 hours per day.

https://enlighten.enphaseenergy.com/pv/public_systems/sntr451420/grid/days

In Tennessee, we figure 1,000w of PV will produce a year average of about 1,000kwhrs for the whole year.....about 2.74kwhrs/day. I can't imagine New Jersey has better solar conditions than here.

quicksandfarmer · Oct 22, 2014

TnAndy said:
Without going thru all your numbers, I can tell you right off the bat a 750w, $1200 PV system WILL NOT produce 5.8kwhrs per day. You are simply WAY off in your numbers there. I didn't wade thru the rest of them.

Assuming the panels produced a full 750w (they don't) for a full 6 hours of sun (more like 2, the morning and afternoon hours will fall off away from 11am to 1pm peak production), you'd only get 6 x 750, which is 4.5kwhrs....and in real world experience, you'd find more like 2.5-3.

In Tennessee, we figure 1,000w of PV will produce a year average of about 1,000kwhrs for the whole year.....about 2.74kwhrs/day. I can't imagine New Jersey has better solar conditions than here.

I'm assuming the heat pump water heater has an Coefficient of Performance of 2.0 (200% efficiency). So to get 5.8 kwHrs of heating takes 2.9 kwhrs of electricity -- pretty much exactly the number you're projecting as a "real-world" number.

This article measured the real-world performance of 14 heat-pump water heaters: Heat-Pump Water Heaters Come of Age | GreenBuildingAdvisor.com

The measured COP's ranged from a low of 1.0 to a high of 2.6, with an average of 1.9. I don't think my use of 2.0 is unreasonable.

dave1949 · Oct 22, 2014

My solar pv installed to generated is within the ballpark of TNAndy's estimate: 4.32 kW system produced 4.99 kWh in one year. My generation averages 416 kWh per month. We have 18 240 watt panels.

416 kWh per month / 18 panels = 23.1 kWh per panel per month.

Our standard 50 gal. electric water heater uses about 120-140 kWh per month (for two people).

140 kWh used by water heater per month / 23.1 kWh per panel per month = 6 panels to power the water heater on average.

Assuming an installed cost of $3.50 per watt: 6 panels X 240 watts per panel X $3.50 per watt = $5040 to heat water for the next 20 years: $252 per year.

That's my experience.

RalphVa · Oct 23, 2014

I tried a direct heat pump to heat hot water. It pays out fairly quickly. However, it won't if the compressors continue to fail as mine did. First time it failed, I got a new unit. This one failed then about a year later. I gave up and reinstalled the electric coils in the tank.

I could not justify the cost of a solar water heater system. The $2,500 rebate only pays for about 1/3 of it now.

If I had the space without their having to run coils in the ground through my woods behind the house (can also put them in wells, but no room for that either), I'd go with geothermal, particularly if you need to upgrade the efficiency of your heat pump system anyway. We, instead, just went with a high efficiency heat pump.

If you have plenty of money to spend, replacing the electricity used with your own via PV panels is the best long range solution. We'd do this if we had another guaranteed 30 years to live to pay it out. We don't; so, no doing this.

I never even thought about using on-demand because they only are practical for gas. If they've fixed the compressor problems with heat pumps in these new hybrid units, it makes sense to go with one of them, maybe in series with an on-demand unit. Just don't turn on the electricity to the coils of the hybrid heater tank.

Ralph

tkappeler · Oct 23, 2014

This is what I love about TBN. Keep the advice coming.

I think my biggest concern with the above calculations, experiences, etc is that none can handle it all. The choices are:

1) Solar thermal hot water in the $1,500 - $3,000 price range but I still need an electric as backup and who knows how much I would have to rely on the the backup in winter.

2) Solar PV in the 4-5 Kw range at a DIY cost of 10-12 before incentive but would still need the same large propane consumption for heat along with wood stove. I could continue to use electric hot water but not address heating

3) Geothermal in the $13,000 - $15,000 range before incentives. It would handle heat and hot water, dropping propane to < 100 gallons a year for stove and dryer but would jack the electric usage.

It seems easy to quantify any one piece of the puzzle but hard to quantify the other pieces since the unknown is how much affect the primary factor would have on the other factors.

Raspy · Oct 23, 2014

The problem, so far, with the arguments against thermal solar is the assumption that it will be a flat plate system with complicated controls and freeze protection. Not necessarily so. You already have an 80 gallon electric WH, which is the most friendly to use since it's not a tankless and not propane. And it's the simplest.

You could build a batch system to preheat the water before it gets to the electric water heater. Depending on how you want to arrange it and if you want to install yourself, it can be very easy. While building a house about 35 years ago, I stripped an electric water heater of its jacket and insulation and painted it with barbeque black paint. We laid it on its side in the rafters in an insulated boxed in area and put a skylight over it. The cold water goes to it on the way to the water heater. it is still working with zero problem all these years later and only cost a few hundred dollars. This could be sized to fit your needs, but the one I did supplies 40 gallons/day, or more, of very hot water in good weather.

This simple system is maintenance free, low cost and invisible to the user as it simply preheats the cold water. No reason it couldn't be an 80 gallon or several 40 gallon units, if needed.

I built my own flat plate system at my home and turned off the water heater for about 7-8 months/year. Total cost for that setup was in the $3200. range and it had a 120 gallon pre-heated storage tank and fool proof freeze protection. Solar doesn't have to be complicated and very good equipment is available.

I don't like tankless water heaters and I don't like propane. The tankless water heaters are complicated, fussy and not that efficient. Sometimes they have advantages, but not very often in retrofit situations. Propane ties you into variable pricing and tank agreements that can be frustrating. Plus propane is more temperamental than natural gas and is more dangerous.

dave1949 · Oct 24, 2014

Using a batch heater is a direction to go for cost and simplicity. It should work just as well ground-mounted as in/on the roof. A well-insulated and vented attic should be only a few degrees warmer than the outside temps in winter anyways. The temperature differential between the ambient air and the water should be very similar in the attic or on the ground.

A ground mounted tank under glass in a super insulated 'box', connected to a pre-heating storage tank in the basement should shave energy costs off of any downstream water heater. Or it can be integrated directly into the COLD supply line to an electric water heater. Easy enough to shut it out of the system with two valves, or drain the batch tank with a boiler drain valve conveniently. Any plumbing mishaps would not cause damage since it is outside.

Ford850 · Oct 24, 2014

Wow, I didn't know hot water could be so complex.

I stick with a fairly basic power vent LP 40 gallon tank. I try to get one with a higher than average recovery rate, since it is a small upfront cost and still rated at the same efficiency to run. I don't think I've ever ran out of hot water, but there are only 3 of us. Power vented models are louder though, and cost close to $1k, but they are relatively efficient to operate. I probably use around $20 per month on LP for it.

techman · Oct 24, 2014

tkappeler said:
3) Geothermal in the $13,000 - $15,000 range before incentives. It would handle heat and hot water, dropping propane to < 100 gallons a year for stove and dryer but would jack the electric usage.

.

A geo thermal HP will not make all of your hot water. It can assist with other systems, but you only get about 1000-3000 BTUH when running. This is less than a KWH. In the winter I can preheat to 90-100 deg, which is good output. In the summer it is only 65-75 deg of preheat.

paul