Photovoltaic to heat water vs solar water heater

[rains808]

Hi - my first post. We may be building a house this year in Hawaii. We're on the west side of the Big Island and get tons of sunshine all year round. I'm wondering if considering the cost of installation & maintenance whether it might be better to use a regular electric water heater in conjunction with Photovoltaic roof panels (net metering system) rather than a solar water heater + collector. In other words, heat water electrically with the power generated from the PV panels during the day. Would need a timer ("little grey box") to switch the power to the water heater off during the evening/night. Also would need an oversized water heater to store hot water overnight. My thinking is that this would eliminate one system: the hot water collector panel and circulating pump and the plumbing that goes with it. Would probably mean a few more PV panels to generate some more KWH to cover the use of the water heater. I hope I explained this well enough - any thoughts on this much appreciated. Aloha, Rich

[Dana]

In Hawaii a combination of PV + hybrid tank hot heaters actually MORE efficient than a thermal hot water, and lowers the cooling load inside the building too.

In heating-dominated climates there's always the issue of hybrid tanks adding to the space heating load, but in cooling dominated climates with high-priced electricity (such as in HI) they're a slam-dunk.

Some independent testing seems to indicate that the larger the tank the more efficiently the hybrid heat pump tank works, but I don't have web-published data on that. The 80 gallon Steibel Eltron Accelera tests better than any of the smaller- tank hybrids out there, and has good first-hour ratings for an electric tank- you can even fill a decent sized soaking tub with it before the resistance elements would kick on.

A couple grand for a hot water heater may seem like a lot, but at Hawaii's electricity rates it'll pay back well within it's anticipated lifetime, since it uses dramatically less power for hot water while reducing your cooling power use at the same time. The installed cost will still be less than that of a fossil-fired tankless, (and less than even an in-line batch-solar hot water heater of lower net efficiency.)

[ballvalve]

I would do the net metering and preheat the incoming water for a basic water heater with some roof mount rig. An uninsulated water tank in a glass box used to be made that closed up at night with insulation - sun runner - Or just a basic recirc rig panel- no need for freeze issues, so its easy.

[Dana]

Roof top batch heaters are pretty cheap, and a better solution than homebuilt hacks like that, but probably a lower ROI than the heat pump water heater approach.

With a heat pump water heater you're really getting as much air-conditioning out of it as you are hot water- you couldn't ask for a nicer fit for net-metered PV in that climate, since most homes there rely on air-conditioning anyway. With cooling loads 300 days out of the year means you don't HAVE to up-size the PV array to get hot water. With any kind of batch solar you'd either have to oversize the sucker 3x to be able to coast through the rainy/cloudy days, or have some sort of backup HW heater. With the solar thermal sized at a more appropriate 50-70% of the hot water load the house would be still be using more power, since there's no AC being offset by the HW heater. At an average room temp of 78F room the COP/water-only of the Stiebel Eltron would be closing in on 3, but the net reduction on power use from the "free" air conditioning it provides would be nearly 2x the savings on hot water heating.

Stated another way: With solar thermal hot water you're bringing heat into a house that already has a cooling load, so any standby or distribution loss from that loop is simply added to the AC load. With a heat pump water heater 2/3 of the heat going into the water comes OUT of the cooling load, and the water coming into the house is never above room temp, so there's no heat being leaked into the house except through the walls of the tank. The only extra heat being added anywhere inside the thermal envelope of the house is the power to run the heat pump, and most of that ends up in the water first, the majority of it going down the shower drain rather than leaking back into the house through the tank walls. It's basically just a little air conditioner constantly sucking heat out of the house, heat that would otherwise be removed with similar net efficiency by the main air conditioning.

[rains808]

Hi Dana,
We have a solar water heater in our current house with an 80 gal - it saves the hot water stored during the morning hours (tends to cloud up in the pm, so we had the collector situated more to the SE) for showers for the spouse and me the next morning. Only time we turn the breakers on is if we have guests. I'll look into the different tanks available (have to see what's here, otherwise pay thru nose to ship a special item).

[Dana]

I'm pretty sure you can get the GE & Rheem models HI, if not the Stiebel Eltron. You can get the the Rheem Ecosense ($1300/50 gallons) through the Home Depot in Kailua Kona without extra shipping, and you can get the GE Geospring (~$1000/50 gallons) through Sears. You'd have to call to see what's in-stock- it's not clear from their websites, but store-pickup at these outlets usually has no additional cost even when it's not an item usually stocked. (I would think that it's a standard stock item in HI though, since cooling dominated climates are the biggest/best markets for these.) I'm not sure if HI offers rebates on them, but there might be some efficiency kick-back available. When there's a choice, bigger volume==better for both efficiency & function- the 50 gallon GE GeoSpring is probably a better choice overall than the 40, etc.

I'm not sure if AO Smith has a local distributor, but the 80 gallon Voltex is another one to look into. It's about $2K from online sources, the 60 gallon Voltex runs ~$1500. The increased performance of a larger unit may be worth it, or maybe not, for a 2-person household. Most of them have an "efficiency mode" where the resistance heater elements are inhibited vs. "hybrid" mode where the resistance elements reduce the recovery time, and only the heat pump runs. If you get something big enough for your own needs it would be similar to your current solar setup- you only run it in "hybrid" mode when the guests are around otherwise leave it in heat-pump-only "efficiency" mode. Clearly 80 gallons works for you, but it could well be that 50 gallons does too, unless you have a big tub to fill. Unlike your current solar setup it's not time-linked to when the PV is putting out, you don't have to adjust your habits or the orientation of the PV array to accommodate the water heater.

[ballvalve]

1000$ to 1600$ for complicated water heaters that have a rather long history of failures seem like a life cycle cost analysis would handily eliminate them over a passive solar and 300$ electric water heater.

[Dana]

A 12 year warranty and reduced AC load in the land of 25 cents/kwh is good enough for me. Clearly YMMV.

The 50 gallon $1000 GE from Home Despot will pay for the $700 difference in price between that and a purty-good 50 gallon standard electric in reduced air conditioning + water heating cost WELL before the warranty period is up- probably in less than 3 years at HI electricity prices even in a 2- person water-sipping household.

How long can that "long history of failures" be, pray tell? (Given that they're ALL new enough to still be within warranty!) Got any references to point to?

Batch solar/passive solar is still not super cheap, about half the installed cost of active solar even for those without freeze protection.

But go ahead, show us the lifecycle analysis of both alternatives, based on 25 cent electricity and a HI location, and DO reference your sources, s.v.p. For simplicity, assume an air conditioning COP of 4 for the whole house AC, and an EF of 2 (rather than the measured 2.3), if that makes the math easier, and assume you replace the hybrid &/or standard tank on a 10 year basis, 20 years for the passive solar.

The fact that the hybrid removes more energy from the room air than it consumed from the power lines means you'd already need at least a 50% fraction from the passive solar + standard electric system just to match it for the hot-water heating power. If the COP of the AC is 2x that of the water heater, the reduced electricity use by the AC for that 50% of water heat is half, or another 25%. The batch solar end would have to exceed 75% of usage to almost-match the whole-house performance of the hybrid, and at those levels you have to design in heat dumping etc to avoid the steam explosions. You may able to support your assertions on the lifecycle cost analysis without some real systems and real data, but the lipstick on bathroom mirror math sez "No way, Jose!"