Method of heating water using solar PV system.

[AlanCam]

I have a solar PV system installed and working. This has a maximum capability of 2.52 kW.
I use Gas to heat my hot water in a storage tank. This storage tank has a standby electrical immersion heater, rated I think at 3kW. Now that I am generating power, which during the day is in excess of my requirements, I wondered if it would be worth it to have the immersion heater switched on during daylight hours to heat the hot water instead of using the gas system.
A number of factors I can see need thought.
1. If I switch on the immersion heater and the power from the panels is insufficient I will be drawing power from the grid. Is it possible to switch on the heater depending on the power being generated?
2. If I replace the immersion heater with a lower powered one, say 1kW, what will the effects be?
3. How can I determine when to switch on the immersion heater?

Sorry that is really 3 questions but I can deal with any answers.

[Mike90250]

Efficiency wise, it's bad, if you want solar hot water, get a solar water heater. many times more efficient than PV for water heating.

Do you have a grid tie arrangement with your utility ? Spin your meter backwards, and get a discount on your bill?

You are generating solar power at your cost of - say $1 KWh. That can unwind your meter and you can avoid electric cost of $0.15 KWh. Over a year, you can "avoid" a lot of cost, if you sell the electric back to the grid instead of superheating your water.

gas water heating is a lot less then electric, unless you get cheap hydro power from somewhere at $0.07kwh

[AlanCam]

Thanks for the opinion. Yes I do have a tie-in. I am getting 0.43 GBP per kWh generated plus 0.03 GBP for each kWh fed back to the grid.
I am in the UK so different conditions apply.

Mod note - You already posted your URL one time.

[PatM]

Dual immersion heater

I'm in a similar position to you and I've been thinking of installing one of these:

http://www.qvsdirect.com/27-Dual-Element-Dual-Stat-Immersion-Heater-pr-18327.html

Unfortunately I recently renewed my immersion heater and didn't put one of these in as I was short of cash, and at

[vinniethePVtech]

A tank less water heater is the most economical and practical. From the standpoint of installation, tankless has far less parts and assembly. Saves up to 1/3 on natural gas water heating, compared to conventional standby tank water heaters. Most solar water heating systems that are installed in the united states are used to heat pools since that is where alot of wasted energy consumption comes from.

If you absolutely want to use your solar electricity for water heating purposes. Look into using multiple tankless electric heaters. The reasons why tankless water heaters are efficient is because heat exchange process is used which apparently is much more effective in gaining rapid temperature, with minimal efficiency losses. You should set it up so there is one electric tankless heater per room. One per bathroom/kitchen. If you don't take long showers you could use less than 2kwh per day.

Electric water heating tanks don't work for PV/ grid electricity as there are more energy losses during standby.

If it were me, I didn't own a swimming pool I would use tankless natural gas.

[russ]

Not totally wrong but poorly explained -

1) A tank or tankless remote water heater system loses substantial heat as the water moves to the consumer point. A 'point of use heater' (located adjacent to a faucet) does save energy. The 1/3 is a number grabbed out of the sky.

2) The part about, 'Electric water heating tanks don't work for PV/ grid electricity as there are more energy losses during standby.'
Makes no sense at all - a well insulated tank has a very low loss of temperature.

Solar thermal waters are often the most efficient use of solar for a home - generally far more so than solar PV

[vinniethePVtech]

Not totally wrong but poorly explained -

1) A tank or tankless remote water heater system loses substantial heat as the water moves to the consumer point. A 'point of use heater' (located adjacent to a faucet) does save energy. The 1/3 is a number grabbed out of the sky.

2) The part about, 'Electric water heating tanks don't work for PV/ grid electricity as there are more energy losses during standby.'
Makes no sense at all - a well insulated tank has a very low loss of temperature.

1.) I grabbed that out of the sky figure of 1/3 because its practical. Most manufacturers of tankless water heaters boast huge energy savings up to 50%. To me that's not practical unless the consumer becomes more conservative for consumption, and installs tankless at each room that demands hot water.

According to this article energy savings are between 27%~50%, according to the installation condition of one tankless per room. http://www.energysavers.gov/your_hom.../mytopic=12820
For homes that use 41 gallons or less of hot water daily, demand water heaters can be 24%–34% more energy efficient than conventional storage tank water heaters. They can be 8%–14% more energy efficient for homes that use a lot of hot water—around 86 gallons per day. You can achieve even greater energy savings of 27%–50% if you install a demand water heater at each hot water outlet.

2.) Insulation is one factor, placement of that water heater whether its placement in a cold garage, or basement matters also. If you have it within the home in warm enviornment most people don't care to use a blanket.

Tankless water heaters just seem to be more superior with integration as it can be used for boosting solar water heating as well.

[Mike90250]

.... From the standpoint of installation, tankless has far less parts and assembly. ...

Huh ?? Most need AC power, they have flow sensors, thermostats, some have power vent fans, all need gas and flue hook-up and inlet and outlet connections. Some brands need manual reset after power failure.

Standing pilot heaters cost 20% of tankless, and are the right choice many times.

uninsulated delivery pipes are a major loss.

[TnAndy]

Standing pilot heaters cost 20% of tankless, and are the right choice many times.

Uh, don't know about that 20% figure.......I just had to buy a direct vent ( vents out the side wall of the basement, draws it's combustion air in the same, dual wall pipe) propane model.....cheapest I could find it anywhere local was over 900 bucks. A Bosch2700ESLP with the vent kit is about 1200 ( cheapest place I could find on the net ).

I went back with the standing pilot for two reasons:

1. My neighbor works in QC at local American Water Heater plant ( and has for 30 years ) and his recommendation was the tankless DON'T save much, and the only reason to install one was to save floor space if that was an issue.

2. I would have had to connect AC line power to the tankless, and THAT was why I got rid of the electric tank heater I had put in when I built the house. After one of our prolonged periods of grid down power ( 2 weeks ) one winter, I decided I was gonna have hot water no matter what.

3. I was sorta leary of taking a fairly simple appliance, and adding a computer to it ( in the case of the tankless versions ) to heat water. My experience is the more complicated something becomes, the more likely it is to break, AND the less likely I am to be able to repair it. So I'm a big fan of the KISS principle.