Why almost no one doing Solar Thermal?

[1201]

You can do it with as little as smart relays, as long as you have enough steps to work with. Ideal way is a static controller that is continuously variable, but I doubt anybody is doing that DIY at home.

How are YOU doing it?

 

[1201]

I have 12.8KW of PV (off-grid) and solar thermal for hot water (Chromagen 50 gallon - thermosyphon). I'm very happy with the system.

Sweet! Where are you located? So no pumps at all for the thermal system?

 

[miahallen]

Sweet! Where are you located? So no pumps at all for the thermal system?

I'm in Belize.
No pumps. Its a single 4x7ft panel. The tank has a 2500W 220V element which operates on a timer switch & thermal switch. So twice a day (evening and early morning), it has the opportunity to come on for an hour and heat if the temp is less than 120F, it works great!

 

[1201]

I'm in Belize.
No pumps. Its a single 4x7ft panel. The tank has a 2500W 220V element which operates on a timer switch & thermal switch. So twice a day (evening and early morning), it has the opportunity to come on for an hour and heat if the temp is less than 120F, it works great!

Wow that's sweet. With a single panel what type of high temps do you see?

 

[miahallen]

Wow that's sweet. With a single panel what type of high temps do you see?

There is a pressure release valve which regulates the max temp to around 135F, even on mildly sunny days it has no problem maxing out. My timer is set to come on at 4pm, but we've only seen the electrical element come on once at that time after 2 overcast days. It comes on 9 of 10 days from 4-5am. It seems the 1 of 10 days it doesn't come on in the morning is when we use less hot water at night (when a family member or two skip a night time shower, preferring to shower in the morning).

So, our hot water seems to always be between 120F - 135F anytime we need it, and we spend on average only 2.5KWh per day to achieve this.

 

[Shimmy]

There is a pressure release valve which regulates the max temp to around 135F, even on mildly sunny days it has no problem maxing out.

Our system gets up to ~160F, which gives good ride-through, although extra insulation would be good. The mixing valve gives us consistent temperature at points of use.

 

[Hedges]

Pv is not that cheap lol. I think professionally installed thermal is just too bloody expensive

Labor for small scale jobs is always too expensive. That makes DIY worthwhile for people almost regardless of salary they earn from day job.

PV panels new are now $0.50 to $1.00/W. Used, $0.10 to $0.50
Grid-tied PV hardware (DIY labor) is around $1.00/W (at least if no RSD required). That works out to $0.025/kWh over 20 years. Approaching half that if cheap used panels.
That is cheap.

PV panels direct to resistance heater would be pretty cheap. Be sure to use DC rated contactor, don't rely on AC thermostats and over-temperature switch to handle DC. Have a temperature/pressure relief valve.

in 2007 ...

However, the system has been trouble free all these years. Do PV inverters last this long?

Yes. I installed 5x SMA SWR2500U 2003 ... 2005
Replaced with other models in 2020 to get other features.

Over the time they operated, had 2 failures. One warranty repair, one after so I took down.
My calculated MTBF is 35 years.

 

[ddeatrich]

With a few cheap panels, a cheap water heater , and a DC relay. You can have hot water for under $1k.
And almost zero maintenance.

Is there a guide for this?

 

[Shimmy]

PV panels new are now $0.50 to $1.00/W. Used, $0.10 to $0.50
Grid-tied PV hardware (DIY labor) is around $1.00/W (at least if no RSD required). That works out to $0.025/kWh over 20 years.

Pet peeve, but it is $0.07-0.08/kWh for 4-4.5 average hours of (rated) sun once you take in time value of money, losses, and tax rebates.

 

[timselectric]

Is there a guide for this?

I'm sure that somewhere there is.
But it's a pretty basic system.
Solar panels feed the heating element, through a DC contactor. The contactor is controlled by the thermostat.

 

[Hedges]

Pet peeve, but it is $0.07-0.08/kWh for 4-4.5 average hours of (rated) sun once you take in time value of money, losses, and tax rebates.

Can you present your calculation?

Last time I worked it out, interest rates were like 0.05%, so time had no value.
Perhaps coincidence, but both rates and price of PV has risen recently.

 

[Shimmy]

Can you present your calculation?

Last time I worked it out, interest rates were like 0.05%, so time had no value.
Perhaps coincidence, but both rates and price of PV has risen recently.

$1,500/kWh(PV) - 30% Tax credit = $1,050 PV
IRR = 6%
FV = 0
n = 240 months
-------- (fancy calculator step)
PMT = $7.52/month/kWh
$7.52 / 30d / 4.5h = $0.055/kWh (PV)
$0.055 / 0.85 AC/DC Ratio = $0.065/kWh AC

I originally took additional 10% losses on top of that, but I forgot that we weren't talking about a battery system. At 7% IRR it works out to $0.07/kWh AC.

 

[Hedges]

I keep my math as simple as possible, avoid "Rule of whatever", approximate area of a circle as 4 r^2, etc. Something else will swamp out error from my simplifications.

1.06^20 = 3.2
That's probably a big part of difference between your financial analysis and my simple amortization. 3.2 x $0.025 = $0.08, close to your 0.065
The investment is up front, while kWh benefits accrue over the 240 months. Best to assume no value at the end.

Other point of view is payback or break even compared to utility rates, which can be quite short for those of us paying $0.25 to $0.50/kWh.

 

[Shimmy]

Other point of view is payback or break even compared to utility rates, which can be quite short for those of us paying $0.25 to $0.50/kWh.

Of course I agree with this; I only point out the "technical" price because when people expect savings from an investment it is important to have those expectations work out.

Philosophically, I think TVM calculations are the key to financial success, and an important way to avoid some of the traps people fall in...

Also, if you are paying $0.25/kWh, a $1,500/kW investment in PV will net you a 32% return on investment (up to the limits of self-consumption or net metering). More people should be investing in PV!! (Even when you add a battery in to cover half your use that incremental investment has an ROI of over 18%.)

 

[sunsurfer]

This is a great topic.

I researched off and on for years on a cheap/reliable/low power DIY way to do thermal with water based medium. Never found anything. Material strength with high temp is not cheap.

In the end, there was always a compromise. If a home is built more around it, there is more wiggle room.

Air based thermal removes allot of issues. I did some designs on paper with an air over water that showed lots of potential and flexibility but never worked out how to move the air through with lower power levels. Water is so much more efficient to move around.

 

[sunsurfer]

I actually thought about adding pex tubing loops under my panels for this reason. Pulling the heat away from the panels would also make them more efficient. Plus it could be used to melt snow in the winter.

Interesting idea. You could use some of those pex aluminum heat spreaders that is used for radiant flooring.

 

[Daddy Tanuki]

My neighbor has a massive system and experienced a leak a few months ago. Sounded like a waterfall and it drained his pool. Flowed into my yard and many others. No thank you. Many other neighbors here in Vegas have leaks with their system as well.

well you should thank him for watering your lawn for free.

 

[SteveyG]

Then I tried ac and dc elements. Again not happy with it since I couldn't tell it really helped any.

This doesn't make much sense, a heating element it just a resistance - there's no AC and DC elements. You do however need an MPPT controller otherwise you'll just be loading the panels with a fixed resistance.

 

[Crowz]

This doesn't make much sense, a heating element it just a resistance - there's no AC and DC elements. You do however need an MPPT controller otherwise you'll just be loading the panels with a fixed resistance.

Back in the day when someone said "DC heating element" for a hot water heater they were referring to low wattage elements.

 

[1201]

$1,500/kWh(PV) - 30% Tax credit = $1,050 PV
IRR = 6%
FV = 0
n = 240 months
-------- (fancy calculator step)
PMT = $7.52/month/kWh
$7.52 / 30d / 4.5h = $0.055/kWh (PV)
$0.055 / 0.85 AC/DC Ratio = $0.065/kWh AC

I originally took additional 10% losses on top of that, but I forgot that we weren't talking about a battery system. At 7% IRR it works out to $0.07/kWh AC.

How do you come about the $1500/kWh?
Also what's the 6% irr number from? Thanks