PV powered On Demand Water Heater ????

BobM

Kicking around the idea of using some spare PV panels I have to power a small on-demand water heater to heat my pool. Doing a quick internet search earlier PV powered hot water is considered a bad idea (due to cost I think). However, I don't need hot water all of the time since it's for a pool and I don't need to reach the typical 120 degrees, shooting for about 80-85. 
I currently have 600w of PV, a xantrax power hub and charge controller. I just don't know if 600w will cut the mustard for even the smallest heater. 
The goal would be to increase the water temp by 10-15 degrees over 7-10 days. 
Here is the thread I found earlier,
https://forum.solar-electric.com/discussion/13188/i-want-to-run-my-water-heater-on-solar-power-newbie

So what does everyone think?

Thanks for all of your help!

-Bob

mike95490

A pool cover will heat more than 600w would

Estragon

A couple of numbers needed to figure this out.  First the easy one; energy reqired to raise water temp by say 10°.  Water weighs ~ 62lb/cu.ft.  so length x width x depth x 62 x 10° = btu required, ÷ 7days ÷ sun hrs = btu/hr. to raise temp.  600w is 600x3.4=~2000btu.

Eg. 10x20x5' pool =1000cu.ft. @62# = 62,000# x 10° = 620,000btu.  10 days x 4 full sun hrs x 2000btu = 80,000btu, enough for only 1° or so.

The second number is tougher; net external heat gains/losses.  This will be a function of delta between water temp which would increase as the water warms) and ambient air/ground temps, insolation, convective/radiant gains/losses, etc.

A 10x20 pool is roughly 3m x 6m, so 18sq.m.
STC solar insolation of 1000w/sq.m or 3400btu x 18 = ~ 60,000 btu.  If a dark pool cover could absorb half that for 4hrs that's 120,000btu/day in gains, vs ~8,000 for pv.  The pool cover would also reduce losses at night.

My arithmetic may be off, but it looks to me like the cover wins vs pv.  The pv might be more useful running a circ pump during the day to make the cover heat gain higher (by reducing stratification and increasing delta between warm cover and adjacent pool water).

FWIW

solar_dave

Another way might be to use the solar panels to run a small pump and get some solar hot water collectors.  That and a cover and you might have a nice toasty pool.