Hi,

For my new place I'm trying to decide how to do the hot water.

Seems like I can do a traditional solar water panel - from which you can choose between various products (I'd be inclined to an EV tube panel; not sure about whether pumped or syphon).

Or I can try to heat my hot water with "excess" power from my proposed PV system.

Here's an article that claims its cheaper to do it with PV (though maybe this is a "temperate climate'" view?)

http://www.greenbuildingadvisor.com/blogs/dept/musings/solar-thermal-really-really-dead

The "winning formula" seems to be a nice efficient heat pump powered by power from the PV system.

By the way - this article links to a nice site PVWatts which is a US Department of Energy predictor for generation.  US developed but it works for us too.

So - what say the gurus?

Thanks,
Elbow

 

 

We have two schools of thought in these parts, if you look at old threads.

There are those like myself who think that a traditional solar water heater remains the best option. What your PV panels make (for the same roof area) this week... the solar water heater makes that by lunchtime. In terms of pure bang for buck, it is incredibly hard to beat even in the Western Cape. You use hot water every day, you want to heat your average "baseline" as efficiently as possible.

Conversely however, lots of people put up PV arrays that are big enough to cover their peak consumption, and therefore have lots of surplus left. Those people might want to rather dump it into hot water. The argument goes that it is much more flexible to have your water heating capacity available as electricity for those times you want to run air conditioners or welding machines instead.

You could also use your PV to power a heat pump. If you do nothing else, you should at least do this: The heat pump gives you a 3:1 coefficient of performance, which counteracts some of the inefficiency of PV panels, and also helps you save when there is little sun.

To me it seems the obvious answer is somewhere in the middle. Thinking back to growing up on the farm, where we sometimes had one bath of hot water and everyone shared it (what luxury!), I'd say you want to heat at least one bath of water using a conventional thermo-siphon rig. My suggestion would be to use something like the HeatBox, which is a solar water heater specifically designed to prefeed another hot water cylinder. Then you can use your PV to make up the difference, either using a heat pump or an electrical element.

9 hours ago, plonkster said:

We have two schools of thought in these parts, if you look at old threads.

There are those like myself ............

That Plonky was one of the most succinct explanations ever. That deserves to be a "pinned" thread.

I cannot see the first post in this thread. The forum keeps cutting it to two lines. Clicking go to this post just reload the thread and then cut it to the first two lines again...

I have looked into possibility of running a heat pump from PV for extra heat on the days that the sun shines (Give PV power) but not hot enough to heat the geyser through EV tubes to 55C.

Taking into account that a Heatpump cost R10k and upwards it almost seems more viable to spend the R10k on another 1KW of Panels, change the element to a 1.5kw one and run it directly from PV.  You can then use the PV power in summer (when EV is enough) to run other things like swimming pool pump/borehole pump.

 

55 minutes ago, Louw said:

change the element to a 1.5kw one

If you do this, just remember it will take twice as long to heat up. It takes around 8kwh to heat a 150 liter tank from cold to hot (assuming a delta of 45 degrees), so with a 3kw element it takes 2 hours and 40 minutes. With a 1.5kw element it takes 5 hours and 20 minutes. Also keep in mind that you have a standing loss of 2kwh per day, the larger element will take 40 minutes to restore the heat, the smaller one an hour and 20 minutes. Since the standing loss is also happening while you're heating, one can roughly estimate that it will take over 6 hours to get up to temp with the small element, and just over 3 with the large one.

In other words, never turn off your geyser when going away for the long weekend. Your wife will not be happy when you come back.

:-)

Voltex makes a dual-heater element. I'd go for that one.

 

On 8/28/2017 at 5:34 PM, Elbow said:

For my new place I'm trying to decide how to do the hot water.

I use the Geyserwise dual mppt with 4 x330watt panels which give me a second geyser/battery store option to be used for other loads of a 24/48v bank.

Edited October 10, 20178 yr by flamegrilled
edit

On 9/8/2017 at 1:36 PM, Louw said:

I have looked into possibility of running a heat pump from PV for extra heat on the days that the sun shines (Give PV power) but not hot enough to heat the geyser through EV tubes to 55C.

Taking into account that a Heatpump cost R10k and upwards it almost seems more viable to spend the R10k on another 1KW of Panels, change the element to a 1.5kw one and run it directly from PV.  You can then use the PV power in summer (when EV is enough) to run other things like swimming pool pump/borehole pump.

 

I did this but opted to use a 2KW element instead of the 1.5KW element. It heats up a bit quicker and I have more space power in the mornings for the washing machine or dishwasher.