Supplemental heating, using a solar box

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kingdragonfly

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#222771 28-Aug-2017 10:25

Until the government forces utilities to pay a reasonable amount to buy back electricity looks like solar electric is not good for return on investment. This website showed at least 20 years ROI for my home.

https://www.energywise.govt.nz/tools/solar-calculator/

So on to the subject at hand. I have a split level house, with the living area upstairs.

I was thinking of using an inexpensive solar box to provide some heating.

2 x 4 meters, facing south. At 90% efficiency, it's supposed to produce around 400 watts per square meter or 3,200 watts of heating when it's sunny. My guess on DIY price would be around $1,000.

So far I've come up with these plans, which seem the easiest / cheapest

http://www.builditsolar.com/Experimental/DeepMeshCol/120116Test.htm

Anyone tried something similar?

Calculating optimal angle in New Zealand
http://www.solardirect.co.nz/Solar_panel_collector_angle.html

Aredwood

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#1854460 28-Aug-2017 23:28

How much output do you get when it is overcast? I have solar hot water on my house - selective surface vacuum tube type. 50 tubes on a 300L HWC. In the morning, cylinder upper and lower temp sensors were bot at 16deg. And when I got home - 45deg. Not bad considering the weather wasn't the best today. Even when it it is overcast with light drizzle - they still produce a little bit of hot water. And with enough sun, they will happily get the cylinder up to boiling. I had to install a heat dump system. (sends excess heat to the spa pool). As the temp relief valve kept on dumping the whole cylinder worth of near boiling water during summer. (cylinder temp now limited to max 80deg).

On continuously overcast days, the system typically heats the cylinder from 15deg to 25deg or so. It doesn't sound like much, but that is still a 1/4 to 1/3rd saving on the amount of electrical heating needed.

And solar PV panels on a MPPT charger, will supposedly still provide 10-15% of their rated output even on overcast days.

I just don't think building such a system would be worth it unless you figure out where to buy those special selective surface coatings. (absorbs solar radiation well, but poor at radiating heat) Even when my solar panels are running at high temps, the tubes feel cold to touch.

As for solar buyback rates - longterm wholesale electricity prices are around 7.5c per KW/Hr. So why should your power company pay you more for solar power, compared to power generated by other renewable generation sources? And even today people with gridtie solar get an indirect subsidy via the low use charges, As they pay less lines fees in total, yet their peak demand is still exactly the same. Despite having solar.

richms

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#1854463 28-Aug-2017 23:31

Aredwood:

As for solar buyback rates - longterm wholesale electricity prices are around 7.5c per KW/Hr. So why should your power company pay you more for solar power, compared to power generated by other renewable generation sources? And even today people with gridtie solar get an indirect subsidy via the low use charges, As they pay less lines fees in total, yet their peak demand is still exactly the same. Despite having solar.

Low use charges? What? I pay the exact same rates as before solar went in, only difference is the few c credits I get for the inadvertant export which is usually because something tripped off thanks to the *(&(*&)(^(&* RCDs that I have to have now. Yay for driving home because of a cold aquarium alert.

Richard rich.ms

Aredwood

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#1854486 29-Aug-2017 01:08

It doesn't apply much to you @richms as you still use enough power to still be on standard user rates despite having solar. (from what I remember you have said in other threads). But often people who have gridtie solar, will be on the low user plan. And since the low user plan has artificially cheap fixed fees combined with an artificially high unit cost. Having solar on a low user plan means that you get a bigger saving than what you otherwise would.

These low user subsidies also mean that power companies must make the standard user plans slightly more expensive. As they are not allowed to offer any pricing plans that will give a lower overall bill than the low user plans. For someone who uses less than 8000 units per year. And since gridtie solar doesn't reduce peak demand, you end up with lots more people on low user rates, as they have low power usage averaged over the whole year. Yet they still have really high peak demand on cold winter's evenings.

As for the Aquarium heaters, try running them through an isolating transformer. Im guessing that there is a little bit of earth leakage through the elements that is tripping the RCDs.

Im dreading the inevitable switchboard upgrade that I will need when I get the new underground mains cable installed on my house. Again because of RCDs, which will always trip on me as I have some really long cable runs downstream of my switchboard. And even worse it is on the diagonally opposite corner of my house from the kitchen.