Solar (thermal) HWS vs PVs

Post by **acmotor** » Mon, 05 Aug 2013, 07:30

A friend's solar (thermal) HWS just died and I put the case that they should just fit a storage electric unit for <$1,000 and spend the other $3,500 on more PVs (another 2-3kW or so) (replacement solar HWS was quoted at $4500).

This in a way suggests the death of Solar HWS.

Thoughts ?

Post by **Hippie403** » Mon, 05 Aug 2013, 13:16

Perhaps a heat pump style unit rather than a jug element type?

Post by **Tritium_James** » Mon, 05 Aug 2013, 14:32

The PV might achieve 18% efficiency, and a heat pump HWS might have a COP of 3.5 (ie pumps 3.5kW of heat into the water for 1kW of electricity use) so overall the PV + heat pump is probably less efficient at heating water than direct solar is.

But it's sure a lot more flexible! If you don't need the power that happens to be getting generated right now by the PV to run your HWS it can go into running your house, or exported to the grid. It's a bit hard to export hot water!

Post by **coulomb** » Mon, 05 Aug 2013, 18:46

Tritium_James wrote: The PV might achieve 18% efficiency, and a heat pump HWS might have a COP of 3.5 (ie pumps 3.5kW of heat into the water for 1kW of electricity use) so overall the PV + heat pump is probably less efficient at heating water than direct solar is.

Right, but since sunlight is free, this is more or less a "space efficiency". If you have suitable spare space on your roof, the effiency cost is just space on your roof that was otherwise unused.

Granted, since not everyone has suitable spare roof space, the efficiency consideration is more important for some situations than for others.

Post by **TooQik** » Mon, 05 Aug 2013, 20:04

When my HWS goes I'll be investing in a evacuated tube solar hot water system like this http://www.apricus.com.au/products/introduction.

Which after posting is probably what you're talking about as having failed....

Post by **Richo** » Mon, 05 Aug 2013, 21:10

I had similar thoughts but for GAS as I have gas heating.
Sure gas is cheap but I can really only use it to heat.

The heat pump would be more efficient but the initial cost would blow it out I think.
I guess it all comes down to time to payback on the different systems vs initial cost and lifespan.

Do we have any numbers for running costs?

Post by **acmotor** » Mon, 05 Aug 2013, 21:50

Interesting comments guys.

One of my thoughts was that a solar thermal HWS was a long way from 100% efficient at times TJ. In particular it suffers when ambient temperatures are low, there is freezing, it is overcast, it is raining and with all that is has a poorly insulated tank that is placed (with most systems) to suffer max wind chill. Even if it was highly efficient it struggles to achieve temperature differential as required for HOT water. Raising water from 10 deg C to 20 deg C even if done so efficiently is just not hot water !

PVs work well when cool and still provide reduced output but still actual power. You can add more PV and get hot water in the end but adding more solar thermal HWS collectors (that then need to be shaded in summer) may still not create the temperature differential for hot water, just give more 'less cold' water.

The heat pump from PV generated power is a good plan, hadn't thought of that, but I guess I was exploring the economic reality that PV + storage electric was cheaper than solar HWS and heat pumps etc.
Heat pumps still suffer from evaporator freeze in cold ambient temperatures. Not an issue for PVs.

Good point that the excess from a PV based system can be used for other tasks or exported unlike the 'waste' of solar thermal HWS in summer.

Shame GCIs are so efficient, you could water cool them.

Post by **TooQik** » Mon, 05 Aug 2013, 22:23

This is an interesting read on the subject...

http://www.ata.org.au/forums/topic/2165

Post by **carnut1100** » Tue, 06 Aug 2013, 05:48

I'm getting PVs installed shortly then when my Dux water heater dies I'm getting gas boosted evacuated tubes

Post by **acmotor** » Tue, 06 Aug 2013, 06:03

Do consider spending the $ on extra PVs.
I'd be thinking Hobart is not a good place for a solar HWS. The gas booster is smart though as it will be needed.

Post by **carnut1100** » Tue, 03 Sep 2013, 15:57

In my case I squeezed in just before cut off date for the 1:1 feed tariff. All New installations in Tassie get 8c feed in while retail is 28c... I get 1:1 until Jan 1st 2019 but if I expand my system I getc transferred onto the 8c feed in....so going for more PV is not an option.
My hot water is a third of my bill...And instant gas is around a quarter of electricity for hot water. Going gas will cut my bill a lot and adding solar tubes to preheat will cut it more.

Can't wait for
my panels to be installed....got the contract locked in before deadline but with the rush to get in before the cut off I enot have the panels up until late October.
5kW should make a nice dent in my bill...especially as I'm part way through fitting LED house light.

Post by **acmotor** » Tue, 03 Sep 2013, 17:31

Sounds great carnut !

Do you find that PV / WT /EV are all special feelings that the 'unclean' just don't get ?

Hey, in Tas, if you have PV, do you still have TOD (time of day) metering (off peak,peak,shoulder etc) or are you forced to a flat rate (28c?) all day ?

Post by **carnut1100** » Tue, 03 Sep 2013, 21:21

Not sure if you can get off peak anymore or not....think they canned it.
If you have pay as you go power then it is charged by time of day but you pay more to have this.
We have a lower tariff for hard wired heaters (called Hydroheat) and one for hot water systems.
My old Rheem system accounts for half of my power use but a third of my bill.
As of now the PV only winds the main meter back, nothing comes off the hot water.
This is set to change and when I go non electric will be a moot point anyhow.
Also next year we are getting power company deregulation and competition.
We used to have just the Hydro Electric Commission here. That got broken up when we joined the national grid with the Basslink cable. We currently have Hydro generating power, Transend who own the lines and distribute the power, and Aurora Energy who buy power from Hydro, pay Transend to move it, add a profit then sell it to us.
All are government owned but in Jan Aurora will be sold and the market opened.
Five years later all 1:1 feed in will be scrapped, and those on it now must maintain an aaccount with Aurora to keep it, which cannot be transferred. If I sell my house the New owners will get 8c.
So I have a tad over five years of 1:1 and the system will be fully paid for before it runs out, so at that point I will be installing a battery pack that holds a day's generation so I only sell cheaply what I genuinely cannot use and I only buy when my pack hits 80% DoD which should never happen....

And yes, it does give me a warm feeling....but as our power here is almost all hydro it's not like say Vic with Brown coal...

Now I want a wind turbine.....not that I need one, but I've wanted one since I was ten....

Post by **acmotor** » Tue, 03 Sep 2013, 23:10

....and a small hydro generator

Post by **carnut1100** » Wed, 04 Sep 2013, 01:17

Trust me, if I Had a creek.......

Post by **bga** » Wed, 04 Sep 2013, 16:03

There is a good value suplier of evacuated tube solar water systems:

RunOnSun

in the sydney area and ships Australia wide. A friend has had one of these for severalyears with good results.

I like the idea of PV solar, but the numbers aren't so good:
Fundamental facts:
Specific heat of water 4.87 Kj/Kg/K
Resistance water heater element: 2400 or 3600 watt
Standing losses in a typical Rheem electric HWS: ~3000 wh/day (10.8MJ)
Solar day approximately 4.5kwh/kw in cooler months.

The scheme:
Make up a 5 series x 2 parallel panel set and feed the element directly with an MPP (area=~13 m2). This will supply approx 9kwh per day on the cloudy months. or 32.4MJ. Approximately 23MJ is available to heat water.
Assuming inlet temp is 20C, outlet is 80C, this can heat approximately 80 litres per day.
Add a heat pump (COP = 2.5) and the total hot water could be 200 litres.

Compare this to a 30 tube collector, width ~2.4 metres, length approx 2m = area =~ 4.8m2. Collector efficiency approximately 75% (guess) => 16kwh per day, so 13kwh heating after losses or 46MJ or 160 litres from 20 to 80C and much smaller roof area needed.

RunOnSun's price for the above with a 250L tank is $1400-$1600 according to their web site. -- a lot better than Apricus lotsa $$$$

Of course, no mater ho large the tank, a couple of teenagers can easily consume all of the hot water

Post by **Richo** » Wed, 04 Sep 2013, 20:33

carnut1100 wrote:
So I have a tad over five years of 1:1 and the system will be fully paid for before it runs out, so at that point I will be installing a battery pack that holds a day's generation so I only sell cheaply what I genuinely cannot use and I only buy when my pack hits 80% DoD which should never happen....

Now I want a wind turbine...

What about putting in a Transfer switch and using an inverter to run the house from the battery, solar + wind?
Then you can also "stick it to the man"

Post by **carnut1100** » Wed, 04 Sep 2013, 21:19

That's the ideal setup....but I Think stage one is a pack that banks my excess by day and feeds by night, stage two is upgrading that to a pack that can run the house for a couple of days and aadding a turbine and maybe more panels.
Then I can switch off the grid entirely....

Post by **acmotor** » Thu, 05 Sep 2013, 06:16

bga wrote:
The scheme:
Make up a 5 series x 2 parallel panel set and feed the element directly with an MPP (area=~13 m2). This will supply approx 9kwh per day on the cloudy months. or 32.4MJ. Approximately 23MJ is available to heat water.
Assuming inlet temp is 20C, outlet is 80C, this can heat approximately 80 litres per day.
Add a heat pump (COP = 2.5) and the total hot water could be 200 litres.

Compare this to a 30 tube collector, width ~2.4 metres, length approx 2m = area =~ 4.8m2. Collector efficiency approximately 75% (guess) => 16kwh per day, so 13kwh heating after losses or 46MJ or 160 litres from 20 to 80C and much smaller roof area needed....

bga, I'm just wondering what you have compared here ? Reality suggests dreamware if a solar thermal HWS claims to collect 16kWh/day with 60 deg temperature differential on a cloudy winter day.
I missed something too. how did 32.4MJ become 23MJ ? Some inverter efficiency or something ? A resistive heating element (like an electric kettle) is 100% efficient at converting electricity to heat. Please explain.

edit: and yes, heat pump still puts the PVs ahead of solar thermal HWS. It is just that extra PVs may cost less than the heat pump, be quieter and have greater life expectancy and work in lower ambient temperatures where heat pump would freeze up.

Post by **offgridQLD** » Thu, 05 Sep 2013, 14:03

A cloudy dull day will be an issue for both PV and Solar thermal solar.

Our House is completely Off grid and for now we just use instantaneous gas hot water. Not ideal but that's what the house came with. A 40kg bottle of LPG is about $110.

Its easy to say just throw more PV up and power a 3000w element in a tank. All good PV is under $1 watt but typically off grid your converting the pv's low voltage output through a inverter and that has a limited load capacity. Accommodating an extra 3kw load could be a big chunk of your inverters capacity.

I guess a separate dedicated string of pv at higher voltage could power the element directly and i guess that's what you guys are suggesting. But all that extra PV wouldn't be getting used when the water is hot. Shame to have that PV sitting idle all the time in summer. It could be powering a AC unit on a hot day.

A great way to take advantage of PV and use the power more or less how ever you like is use something like the Australian made Selectronic sp pro line of inverters that can do AC coupling.They are a great bit of gear and open up a lot of options when integrating battery's, grid tied PV and inverters with low voltage PV, generators and battery charging all at the same time. They really are a great fully programmable central power hub that you can feed all kinds of power and have it distribute it the way you want it . AC output, DC charge, coupled what ever you need.

http://www.selectronic.com.au/sppro/

Our plan for hot water is to keep the two instantaneous gas units we have now (long house so need two units) and just feed them water from a two simple vac-tube units.The old gas units will just flow through and wont kick in unless the water is under 50C. It's the simplest way to go about it using what we have and achieving effective gas boosting if its needed.

You can never have to much PV though

we have over 8000w and could end up with more down the track if we install a swimming pool.

Kurt

Post by **bga** » Thu, 05 Sep 2013, 20:04

The numbers are a bit of fiction

It was more about a magnitude thought experiment.

The storage tanks lose a lot if outside. My best guess is 3kwh per day. ~10MJ lost through the tank insulation!

One issue would be the size of the collector needed. Again, I guess approximately 1/4 if direct heating of water is used.

I have one of the RunOnSun 18 tube units waiting to be installed. I should be able to get some good numbers from it eventually.

Post by **bga** » Thu, 05 Sep 2013, 20:05

Summer is always a problem for PV - all batteries charged by 10AM and nowhere for the excess energy to go, save running an airconditioner.

Post by **offgridQLD** » Thu, 05 Sep 2013, 20:12

We are in the same boat so usually on float by 9 - 10AM at the latest in summer NE facing PV. Charging an EV sure gets the charge controller fired up again after 10AM and a place to put the excess energy.

Kurt

Post by **Richo** » Thu, 05 Sep 2013, 20:39

Just use excess power to pump water up a hill or something.

The AC inverters are cheap compared to the whole system so adding another on a separate circuit is a no brainer.
Esp since most houses have 2-3 circuits as a minimum anyway.

Post by **Vectrix150V** » Wed, 11 Sep 2013, 17:24

So Tassie went to the 8c feed-in tariff as well - we've got the same here in NSW, so makes sense to put the power to use somewhere.

Would be great if you could control a gas storage HWS like you can an electric. I'm still trying to figure if there is anything clever I can do - when it dies I think I might go to instantaneous gas.