PV for DHW

solsean · April 2019

I've been seeing PV being used in the European market for almost half a decade to heat the domestic water. The controller I would like, Resol DletaTherm PV, is not approved for US. Is there a controller in our market that can shunt the excess production to the DHW? I'm familiar with the Sunearth product but I would like to do a dual fuel tank for recovery reasons.

Jamie Hall · April 2019

PV is sexy and cool. However, a properly designed and installed direct thermal hot water collector, heating domestic hot water, will give you five times the heat per square foot of collector vs. photovoltaics.

Royboy · April 2019

I'm a long-time solar thermal installer with a 34 year old radiant slab heating system (polybutylene tubing!) and a 96sf solar thermal system. and I'm about to pull the trigger on a large PV array primarily to offset the electric resistance (off-peak) heat that goes into that slab. in my northern WI climate, there's just not enough sun during most of the long heating season to do much and no practical way to store thermal energy seasonally. PV and the buy-back program from my utility provides what is essentially an annual storage system for solar energy. around here for summer, or even 3 season, thermal loads, a thermal system is more productive. but with PV prices as low as they are, PV for winter thermal loads is gaining traction.

Jamie Hall · April 2019

@Royboy makes some good points regarding solar thermal and solar PV -- although solar thermal can, in new construction, provide -- if not 100% of your heat load, pretty close to it, even in evil climates like northern Wisconsin.

However, with PV and considering the use of the utility grid and buy back programs, you should be aware -- very aware -- that you are making a politically based analysis, not an engineering one. That may or may not matter to you...

Alan (California Radiant) Forbes · April 2019

Jamie Hall said:
PV is sexy and cool. However, a properly designed and installed direct thermal hot water collector, heating domestic hot water, will give you five times the heat per square foot of collector vs. photovoltaics.

Jamie: I don't see many thermal DHW installations here, but when I do, they have usually been abandoned because they seems to always break down and need repair. People give up on them. Because of this, I don't recommend solar thermal.

I know they do it well in places like Israel and elsewhere, but I haven't seen it here.

Jamie Hall · April 2019

Agreed, @Alan (California Radiant) Forbes -- they have to be rather carefully thought out and designed -- and then built properly and cared for. Either main flavor -- drainback or constant. Biggest problem seems to be failing to account for the fact that -- particularly for the constantly filled versions -- if circulation fails they can get... um... wamr? But even without that, expansion problems can and do cause a lot of trouble.

They are definitely not in the slap some pipe in a glassed enclosure on the roof ("here -- hold my beer while I put this up") category.

Larry Weingarten · April 2019

Hello, I posted this once before, but it's relevant here too. I have a website and blog here: http://www.larryweingarten.com/blog . If you scroll down to the post on 12/30/18, you'll see an article on a different type of solar thermal. It cannot overheat and freezing doesn't bother it. It sidesteps many of the problems of conventional solar. Anyway, It might be one answer to the question of solar thermal vs PV plus heat pump.

Yours, Larry

hot_rod · April 2019

> @Jamie Hall said:
> Agreed, @Alan (California Radiant) Forbes -- they have to be rather carefully thought out and designed -- and then built properly and cared for. Either main flavor -- drainback or constant. Biggest problem seems to be failing to account for the fact that -- particularly for the constantly filled versions -- if circulation fails they can get... um... wamr? But even without that, expansion problems can and do cause a lot of trouble.
>
> They are definitely not in the slap some pipe in a glassed enclosure on the roof ("here -- hold my beer while I put this up") category.

curious how many ST systems you have owned, installed or serviced?
Any first hand experience?

considering Sun Earth and Heliodyne have been building and selling in California for 30 plus years, must be a market? More often than not collectors outlast shingles.
Rare to find a quality coper collector that will not outlast many mod cons these days 😀 Even the top brand German boilers

Jamie Hall · May 2019

In direct answer to your question, @hot_rod , very few. There are, in fact, very few in my immediate vicinity. I have, however, designed and had built about a dozen 100% passive solar homes in New England, plus a dining hall and an aircraft hangar, and I know of quite a few more. My brother-in-law, on the other hand, has six houses with solar collector type solar heating, rather well done (I didn't do them), and they have stood up well to the weather. Unfortunately, they do not manage the heating load under real winter conditions...

At the risk of pointing out the obvious, though, what works in California just may not be completely appropriate in New England. Although I will admit that the worst we had this winter was a week with no sun, temperatures staying below 0, and three days in the middle of it with 40 plus mile per hour winds... (peak gust measured at Cedric's home was 77 mph).

hot_rod · May 2019

My experience is the failure in collector piping is due to lack of glycol maintenance, ph drops, fluid becomes aggressive to metals, pinholes, sludge,etc. Drainback eliminates that concern

Thermal expansion is handled with proper piping practices, as applied to steam, fin tube installations, any piping
That sees temperature change will “move”

Expansion within the collector is handled with flexible grommets in the case to allow movement both directions

Collectors are brazed and easily handle 300- 350F stagnation temperatures. All collectors go into stagnation conditions when flow is stopped, mostly when the load is satisfied. Piping and or control functions, dump zones can make that a non issue

Controls are much, much simpler than hydronic boilers😙

serpentine style absorbers have no soldered joints, just a loop in and out

If the expansion is not accounted for, the tube arcs in the collector just like a long length of fin tube when the ends are constrained

A savvy experienced pipe fitter or plumber can/ should be able to install a long lasting ST system

Like any trade, unqualified installers sour the industry. Most all the ST manufacturers provide installation training courses

jumper · May 2019

DHW is best use of solar energy because it's relatively easy to store and control the harvested energy. Simplest and least expensive storage is thermal. To keep tanks reasonably sized we need extra hot water, say 200° or even 250° or even hotter.

PV resistance is simpler and (I think) less expensive than a concentrating thermal collector. PV is supposed to be efficient at cold temperatures whereas thermal works otherwise.

Some time ago I angered some people here by declaring solar thermal defunct.

PV for DHW

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