Why almost no one doing Solar Thermal?

[kennyb]

I see I'm late to this party. My houses original heating design depended on a solar thermal/bio thermal system. Sadly the recent epa changes made the manufacturer stop importing - walltherm was the company.

In addition to that the sheer cost of a solar thermal system vs pv and a heat pump dhwt make choice a no brainer. The rheem HP phpro uses 500w of power to heat water in heat pump only mode. The load of the hp is easily handled by 2 extra PV panels on an array.

Simply put in my experience, wires are easier and cheaper to run than piping, coolant, pumps, indirect storage tanks and specialty controllers.

 

[740GLE]

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

who wants to heat your floor in the summer?

 

[hwy17]

I will staunchly defend pool solar thermal over PV-HP to the end of the earth. It is a far more efficient use of resources and yes we are in this frenzy of production that makes these mining and manufacturing intensive processes somehow competitive, but it's clear as day to me there is better resource value in solar thermal.

For $1000 of plastic extrusions and moldings I have a 160sqft array that provides something on the order of 20-80kwh per day of heat into my pool in the summer months for only the ongoing cost of staying on top of leaks and a relatively minor increase on pumping energy required as compared to that which is already required just for filtration (I run 2 extra hours a day for solar compared to what I would just for filtration, and the increased head pressure lowers the pumps energy consumption a bit anyway).

 

[740GLE]

Are people heating pools this much? I’m used to swimming in lakes and rivers in the northeast, if water temps climb above 70 degree it’s getting too warm, I want to feel refreshed, not in a hot tub.

Who doesn’t like swimming in the 60 degree ocean?

 

[hwy17]

Are people heating pools this much? I’m used to swimming in lakes and rivers in the northeast, if water temps climb above 70 degree it’s getting too warm, I want to feel refreshed, not in a hot tub.

Who doesn’t like swimming in the 60 degree ocean?

You get spoiled so fast. Sure I will still get in the 60 degree ocean here, but I won't look at my pool until it's over 80 and I prefer 86 on an average summer day for hanging out.

Athletic pool regulation temp is 78-82.

 

[Hedges]

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

He meant $1500/kW
GT PV hardware is about $1.00 to $1.50 per W.


6% or 7% was a target rate of return on his investment. He didn't want to accept my amortized $0.025/kWh which assumed time value of money was nothing. He could have assumed $0.07/kW and got some rate of return. Instead he assumed a rate of return he could accept, and calculated what cost per kWh of produced power that implied.

When he plugged in my utility's electric rate of $0.25/kWh he got more like 30% rate of return. A no-brainer, assuming you can put all power you generate to work or export for 100% credit.

A coworker once asked me if it was worth participating in the employer's stock purchase plan, which gave us stock at a discount off lower of either starting or ending price for the 6 month period. I told him, "Beg, borrow, or steal if needed to survive so you can participate." The worst you could do in that program was get 70% annual return on your money (assuming you sold day after purchase, and price didn't crash). If price was higher at end of 6 months, you got even more.

Internal Rate of Return (IRR) Rule: Definition and Example

The internal rate of return (IRR) is a metric used in capital budgeting to estimate the return of potential investments.

www.investopedia.com

 

[1201]

He meant $1500/kW
GT PV hardware is about $1.00 to $1.50 per W.

Oh I see. Is it really that high for diyers?

Grid tied growatt 6000 $750
6000 watts of panels $3420
Roof mounting $1000
Misc $1000 ???

Before tax 6170
After 30% credit 4319

4391/6000 =720/kw

Am I missing some line items?

 

[Hedges]

$6000 for 6kW, $1/W, is the figure I've been using.
Some prices have gone up with recent demand.
Your $0.125/W inverter is cheaper than most. SMA Sunny Boy are around $0.20 to $0.25, but I'm using old models which cost me $0.10. Next install will have to be new for required features.
New panels range from $0.40 to $1.00
RSD has added cost per panel.

Your numbers are valid, as are his and mine. Just differences in component selection.
My $0.025/kWh cost assumes it lasts at least 20 years. $0.03/kWh if one inverter replacement needed.
Your inverter and panels may or may not prove reliable. Some inverters have a track record. Some panels degraded rapidly.
Considering all the work, I look for equipment with a good history.

 

[sunsurfer]

who wants to heat your floor in the summer?

In response to his idea, the spreaders would be for placing under the solar panels to cool them

 

[dcg9381]

I've done thermosyphon water heaters. They are common elsewhere, but not in the USA. Part of the problem is mouting 80 gallons of water at 8 lbs/gallon on a roof somewhere. We had to design for it. What I like about these - no pumps to fail. Entirely passive. We could do about 3 days without any sun before water cooled enough to need aux heat.

 

[1201]

I've done thermosyphon water heaters. They are common elsewhere, but not in the USA. Part of the problem is mouting 80 gallons of water at 8 lbs/gallon on a roof somewhere. We had to design for it. What I like about these - no pumps to fail. Entirely passive. We could do about 3 days without any sun before water cooled enough to need aux heat.

Nice!

 

[Ampster]

I've done thermosyphon water heaters. They are common elsewhere, but not in the USA.

Yes, I have seen them on roofs in Asia.

 

[Shimmy]

I've done thermosyphon water heaters. They are common elsewhere, but not in the USA. Part of the problem is mouting 80 gallons of water at 8 lbs/gallon on a roof somewhere. We had to design for it. What I like about these - no pumps to fail. Entirely passive. We could do about 3 days without any sun before water cooled enough to need aux heat.

Is it an evacuated tube or plate system? Do you have an auxiliary tank inside or just the roof mounted one?

 

[dcg9381]

Is it an evacuated tube or plate system? Do you have an auxiliary tank inside or just the roof mounted one?

Thermosiphon system. Meaning hot water rises, so you put the tank ABOVE the panels and it auto circulates. Roof mounted. Uses propylene glycol as the heat fluid, copper coils in the water tank. Like this:

 

[miahallen]

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.

What is wrong with loading the panels with a fixed resistance?

 

[timselectric]

What is wrong with loading the panels with a fixed resistance?

Nothing, that works too.
An MPPT just makes it more efficient.
Also more expensive.

 

[NwCali]

I've done thermosyphon water heaters. They are common elsewhere, but not in the USA. Part of the problem is mouting 80 gallons of water at 8 lbs/gallon on a roof somewhere. We had to design for it. What I like about these - no pumps to fail. Entirely passive. We could do about 3 days without any sun before water cooled enough to need aux heat.

when I bought my house it had a 6 person hot tub on the deck place was built 1978....it was about 10 feet off the ground (deck was). I tore it down, not a bolt, nail, ,screw, nothing holding the timbers holding it all up except being attached to the house side with nails. I live in Earth Quake territory as well. Nutty that they did it, nuts that it never came down.
I should mention that the deck was also 12 feet wide, 50 feet long, and the hot tub was on the far outside corner.

Nope, just sitting on timbers. Not even a cross brace at all.

 

[SteveyG]

What is wrong with loading the panels with a fixed resistance?

It won't do any harm, but a PV panel can be approximated to a current source, so depending on the resistance of the heater you can be dropping the efficiency significantly.

Think of it at worst case where you have a short circuit - you'll get the maximum short circuit current, but the voltage across the cells is close to 0, so the power will be almost zero too. An immersion heater often has a resistance in the range of 18-20R, so it's highly unlikely you'll be anywhere near the top of the power curve.

 

[Hedges]

You can select resistance based on Vmp/Imp, which is what I did for testing PV panels.
Between 1 and 2 electric radiator heaters (each having 600W and 900W switches) for a 165W "24V" panel.

For this application, better to target lower sun Vmp & Imp, which is when you want the most efficiency.

 

[jrcromer]

Maybe... just maybe.... everone thinking about PV thermal is doing a really bad job at thinking about pragmatic solultions with actual paths to market. Heat pumps are doing amazing, yet solar thermal sits on the sidelines looking like an overpriced, never MVP.