Wanting to get into solar? Think HOT WATER 1

[OperaHouse]

Some may be feeling that they are missing the solar power bandwagon and wondering how they can do something actually useful without spending a lot of money. Batteries and grid tie make a system expensive with a long payback. Creating hot water is the most effective way to use a couple of PV solar panels since close to 100% of the power generated will be used. This assumes you are currently using a resistive hot water tank and not on demand as the rest of the civilized world. Never could get used to climbing into a shower with a 240V line.I have been heating hot water for years with excess PV for years at my camp. With as little as 800WH, we can both get a long hot shower at the end of the day. Just a couple panels totaling 400W can make a significant dent in electric use for heating. Under powering the heater assures 100% of the panels output is used. That is much greater than any battery application and much lower in cost than grid tie which isn't even allowed in some locations. Typical heat loss during the daylight hours of a well insulated tank is more than 1KWH. Solar makes up for that loss.

Most tanks have two heating elements. The top is the primary and only one operates at a time. That allows the lower resistive element to be used for solar and still have grid power backup. At 240V even a 5500W element is 10 ohms, a replacement 120V 2000W is about 7 ohms. That requires at least a 36V panel string toget any useful power. One positive is that may keep the system classified as "low Voltage". Connecting a panel directly to a resistance will result in power losses of more than half. A simple PWM circuit keeps the panel voltage at the the power point. A small capacitor bank stores the panels power in the off cycle. A panels power point voltage is the same regardless of light level. It only varies with temperature. Attaching a couple of flat pack diode bridges to the back of the panel is an easy way to get a reference voltage tracks panel temperature. I use a $5 UNO to control everything and the 490Hz PWM makes driving a FET easy. Almost any switchmode chip would work just as well.

I'm not going to argue the economics of it. Watching solar work is just plain fun and panel prices are so cheap now. Everyone has some space they can stick a few panels on. It won't freeze and none of the plumbing issues of direct solar heating. I've built several versions with just junk box parts. I have more real world details if anyone is interested.

 

[crshears]

Hmmm...

Here in Ontario, Canada we are on time-of-use electricity charges Monday to Friday, and since our residential hot water demand is not great we've wrapped our tank in an insulating blanket for the extra R value, piped in a thermal loop in the outlet line, and wired the power supply through a timer such that the water heater is powered up only from 1915 local time until 0645 the following morning. For the most part this has worked out reasonably well.

The idea of hooking solar panels to the "back" contacts of the timer so they would be in service from 0645 to 1915 daily while off grid would dovetail in very neatly with this scheme...I'll definitely look into this.

Thanks, OperaHouse!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[GTTofAK]

I would think that if you want hot water I would think that collector arrays would be more efficient than PV. A PV cell is just a PN diode. PV has limited efficiency because you only get the amount of energy that it takes to knock off one electron from the N side. All photons under that threshold are wasted and all energy over that threshold is also wasted.

 

[IRstuff]

The most efficient PV cells are still under about 25%, so 75% of the solar energy is simply reflected or goes into heating the panel.

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[OperaHouse]

Life isn't always simple. At my camp none of my solar panels are actually on my property. It can be difficult to find a place close to put solar, adding plumbing makes it even harder. Antifreeze, heat exchangers and pumps add a lot of cost. A home show gave a cost of about $10,000 for direct solar. With PV the installation is relatively low cost wire. I highly suggest a heat pump, my winter home has a NYLE add on that costs about a thousand. All in one HPWH systems cost about $1600. They all have a COP of about 2, regardless of what they say, cutting the cost of electricity in half. Adding PV solar for about $450 is not unreasonable compared to these expenses. My camp is basically battery less (one car battery used only for starting surges). A KW of panels to supply a 50W load. Any excess heats water. A ten and twenty gallon tank are in series giving fast response. The point is nothing gives a bigger bang for the buck with PV than heating water and you have to operate the panels at power point or the power drops to almost nothing.

 

[itsmoked]

I can certainly agree with the less failure prone scheme of PV water heating over heat exchangers and a bunch of lamoid plumbing.

My buddy just put in 6.5kW of panels on his roof. Since Pacific Graft & Extortion doesn't pay for any energy over your annual total consumption he was looking for a way to not throw that excess away. He got a 30gal electric water heater and for starters has put it on a timeclock to avoid time-of-use energy draw. So, between 7pm and 10am he allows his water to be electrically heated. His gas usage has dropped a tier.

I'm not convinced a timeclock is going to be the best moderator for this scheme. I suspect some longer averaging will be needed to account for "dim" months or sporadic heavy electrical use when relatives have been by etc., etc. It could be something like a plc that watches the past days of a month and towards the end of the month starts piling on the pv waterheating to try to zero balance the energy consumption might work better.

I'm waiting for an energy mishandling that has him wailing and scratching at my door. LOL

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

That seems a bit odd from a financial perspective. My typical gas bill is $30, so swapping the gas water heater for electric sounds like a losing proposition for me, since electricity in my tier is running $0.26/kWh. So, if I get, say, 10kWh per day, that would save me about $78 per month, which is way more than my gas bill. Obviously, there's still an issue with supply and demand

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[itsmoked]

I was surprised my friend had so much surplus to start with. Turned out the pv system was designed to also support a large hot tube. The week the pv system went live he was banned from filling his hot tube by California's on-going drought.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

After making it through my first winter with gas (propane ), my daily bill was averaging $23. You can bet I'll be installing another pellet stove before the next winter hits. Still trying to figure out why my electric bill was $500/month when it's gas heat...

Now that summer is here (with the occasional cool day worthy of heat), I can barely see the meter move (cook and heat water with gas, too).

Dan - Owner

http://www.Hi-TecDesigns.com

 

[moltenmetal]

Domestic water heating with natural gas is so cheap here than anyt investment you might make to reduce it, aside from adding tank insulation or simply wasting less hot water, has an infinite payback period. Solar thermal collectors, heat recovery on shower drains etc. are all a waste of money here given that gas is so cheap- even if you count your own installation labour as FREE. Even installing a 98% efficient instantaneous tankless water heater, which I did because we have a large tub which can't be filled from a single conventional hot water tank, will never pay the difference in capital back against a 60% efficient tank water heater. That is, assuming you don't have an open flue chimney on it which breathes out hot air all winter whether the heater is running or not, which is what we had- changing that immediately is a no-brainer in our climate!

Obviously if you have already paid for a solar PV installation and are generating more electricity than you can sell, using it for any beneficial purpose is better than wasting it or disconnecting the panels, and hot water heating is an easy way to do that. That said, electric hot water heating, at our average cost of $0.20/kWh once all taxes and fees are in (that's the average of peak and off-peak rates), is an idiotic source for either comfort or hot water heating on an economic basis when compared to natural gas- unless you're running a groundsource heat pump with the electricity. An air-source heat pump might make economic sense in the shoulder seasons for comfort heating, but that just means you have even fewer days of sensible use per year to pay back its capital cost.

Here in Ontario, Canada, if you can install enough unshaded PV panels to pay back the (substantial) cost of permits, inspections and 2nd meter installation, you can obtain a microFIT contract which will pay you a premium for ALL your PV electrical generation- not just for generation in excess of what your own home uses. That's a spectacularly bad piece of public policy in my opinion- but if you can fit the maximum 10 kW of panels on your roof without cutting down all your mature trees, it can be quite lucrative- if you don't mind stealing from other rate-payers!

I'm contemplating a not-quite-legal small "behind the meter" solar PV installation for peak-shedding, which will pay back over the long term because our peak kWh rates are so high. To do it without "detection", I need a cheap, safe and reliable means to disconnect the panels or to add loads (such as a small hot water heater tank) to make sure I never, ever run the meter backwards- and I'm open to suggestions on the best way to do that. The microinverters I would use would already completely disconnect the panels in the event of a power outage (to avoid injuring service workers).

 

[OperaHouse]

$30 a month for gas, my service charge is $28 a month if I don't use any gas at all. Service fees are actually higher than the actual gas used. Th gas water heater started to leak just a few days before going on vacation. Upon returning I installed an electric one temporarily till my buddy could get a dented gas replacement cheap. It was actually plugged into a 120V outlet so effectively a 700W heater. Years later still no gas replacement. I bought a NYLE HPWH left over from the HOT SHOT engineering study. My average daily cost is 38 cents a day, hard to improve on that.

Smart meters are going to make a major change in energy usage. The utilities finally have a way to slice and dice billing costs into minute segments. Peak energy costs the utilities dearly and now they have a way to pass that on to the actual users. Energy management will be a major growth area. Those in renewable energy are actually bigger energy wasters that than that housewife in Sheboygan.

 

[cranky108]

Maybe you should invest in a Tesla battery for all that extra energy.

 

[VEBill]

The $3500 Tesla battery only holds 10kW-hr worth (about $1, YMMV*) of energy.

I wonder what is its lifespan in charge cycles?

If it's not at least 3500 cycles, then it's fiscal nonsense.

(* YMMV with power rates.)

 

[IRstuff]

Lucky you, 10kWh would be $3.10 for me
But, that means I only need 1129 cycles ;-)

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[cranky108]

At 1200 cycles would be about 4.5 years in commuter service. How long should electric car batteries last?

 

[OperaHouse]

Lead acid batteries work out to about 12 cents a KWH. You might as well be on the grid. I think this form of hot water assist will be commonplace in another ten years. Here is a "commercial" unit that sells for $270. I believe it is a very poor design and do not recommend it. The explanation is sufficient that I don't have to duplicate it.

http://techluck.com/

 

[VEBill]

One design concept that I've seen for the risk of freezing is to have a self-draining solar collector. When the sun come out, and the temperature in the plumbing rises, then a small PV panel powers up the pump. If the sun isn't out, or the temperature in the panel is too low, then the water drains back inside the dwelling.

I've seen commercial panels that used this concept. But I didn't see where the $4000 each price tag came from, being made with about $400 worth or materials. Must be some sort of 'Green Tax', LOL.

 

[itsmoked]

moltenmetal maybe something like this would "shave your peak" :
[URL unfurl="true"]http://solar-trap.com/[/url]

He asked some questions in here:
thread240-368902
thread240-374737

It actually makes some sense for peak shaving without the meter getting involved.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[OperaHouse]

I've been in solar for years and have designed all my equipment. I don't get Solar-Traps's concept at all. As they say, Long on the leaf and short in the can. It can be replaced with a simple voltage monitor. I run my fridge without all that fluff.

The HOT SHOT program I talked about was paid for by the utilities to study the effectiveness of heat heat pumps. Not only total energy savings and can a customer live with slow response, but the effect of reducing peak electrical demand for heating in the morning and evening.

On another site a poster had a concession stand with multiple food devices with heaters. He was only allowed a single 20A outlet by the festivals he went to. His total possible demand was a little more than double that. I thought it was an interesting problem that could be solved with a micro scheduling priorities and not interrupt his normal business. This will be the future of our smart appliances. Wife wants a dishwasher this year (I got a look when I told her I thought I married one) and it is possible with as generated solar. The dishwasher will cycle on and off as sun is available or other priority bumps up.

Just thought I'd throw that hot water heating idea out there and see if it stuck. About the only expense is the panels themselves.

 

[moltenmetal]

The uncritical hype over the Tesla Powerwall has been depressing. The thing comes with a charger but without an inverter. Most of the benefits of Li-ion batteries are wasted on stationary power applications. And there's no economic benefit to storage for people who are grid-connected anyway- it will never pay back as a peak-shaver. There's very little environmental benefit to shedding off-peak grid load either, especially in a locale like ours with lots of nuclear- you can't shut those puppies OFF... For offgrid? Lead-acid is already cheaper once you figure in all the real costs- remember you can only buy this thing from SolarCity, and their services don't come free... Peaks most of the year line up with peak solar production pretty well anyway. Nobody should imagine that the Powerwall is a whole-home UPS for $3,500 either...it's just a channel through which Elon Musk can hedge his bets that demand for his brilliant but extremely expensive cars will not consume 100% of the batteries he'll be cranking out from his "gigafactory".

One fridge isn't going to do much for me, and I only have one. My household draw can drop as low as 600 W mid-day and I'd be putting perhaps 1.5 kW of panel capacity on my back wall- can't put more, so a microFIT contract doesn't make sense for me. Didn't see any details about Solar Trap's means of power monitoring at first glance- will have to do some digging. My idea was much simpler: a couple turns of wire around each AC phase where it comes into the house, with a diode and a cap, running to the input of a couple comparators. Sensed current drops below a threshold and I turn on an auxiliary load (i.e. a hot water tank etc.). Current drops below a 2nd threshold and I open a contactor, disconnecting half the panels. The challenge would be to make it work fast enough- regrettably my scheme won't tell the difference between current coming into the house and current going back out to the grid (what I want to avoid).

As to the question about EV batteries- they can last about 3,000 cycles to something around 70% depth of discharge or less. They do suffer from capacity loss with respect to time at high voltage (sitting fully charged, ready to run) to some extent irrespective of charge/discharge cycles. The primary degradation mechanism, aside from cell reversal which occurs if you don't have a proper battery management system, seems to be electrolyte chemical damage leading to deposition of insoluble coatings inside the cell. Time at high voltage, particularly at high temperature, are both bad news.