Heat transfer for Water Cooled Solar Cell 1

[Kuhuh]

I need to find the additional amount of energy to heat a 50 gallon water tank that is on average 68 F to 120 F. I have 4-2.5" (6 ft of exposure to water)copper pipe that runs down the center of the Water tank carrying 98F warm water from a water cooled PV Cell. System runs at .71 Gal/S. Can anyone tell me the steps to find how much heat the pipes transfer into the system and how much more energy it will take to raise it to 120F I know how to find out from 68 to 120 but not with the heat exchanger in there.

 

[IRstuff]

??? If your PV water is 98°F, then you are NOT heating your hot water heater, you are cooling it. Seems to me that your best bet for the "additional energy" is to cut off the PV water pipe and seal it. You'll instantly get a temperature increase in your water heater with no additional cost. Just think about it; 120°C water in the water heater right up against 98°F water in the PV pipe. What does Newton's law of cooling tell you?

I used to have a solar water heater that supposedly supplemented the main water heater. It sucked, and once I turned off the valve from the solar water heater, I never had cold water coming out of the hot water side again.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[IRstuff]

oops 120°F...

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FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[flash3780]

To heat 50 gallons of water from 68F to 120F, you need to multiply by the specific heat: Q = C*m*dT, or in your case Q = (1BTU/lb/F)(50gal)(8.34lbs/gal)(120F - 68F) = 21648 BTU

That's how much heat you need, anyways. The other half of your problem is a bit more complicated. You'll be fighting against the 98F pipe to raise the temperature of your tank to 120F (it will be pulling heat out). Incropera & Dewitt is a great reference for solving these types of problems.

Obviously a transient problem is tougher than a static problem. I'd start with the static solution and figure out what's required to bring it to equilibrium.

 

[ione]

It seems I’m missing something here.

I don’t mean to restate the problem in a misleading way, so if I’m mistaken please correct me.

The OP said there’s a water tank (50 gallons) that has an average temperature of 68 °F and the goal is to heat it up to 120 °F. There is availability of water at 98 °F flowing from a PV cell, 0.71 gal/s

- So first step evaluate what temperature can be reached in the water tank with the source at 98 °F. There is indeed a ?T which acts as the driving force to increase and not reduce the water tank temperature.

- Second step determine the thermal power needed to from the above temperature (unknown) to the target temperature of 120 °F.

 

[mauricestoker]

For the OP, I would ask how the PV HX maintains constant flow and constant discharge temperature, especially if it would be related to weather data? Is this a strictly hypothetical question? If the tank average temperature is between 68 and 120*, it would appear that you have an additional heating source or the tank is sitting in the open. If the intent is to use PV HX as a preheat for hot water, then a temperature differential switch between make-up water and PV HX water might be applicable.

If you have a constant PV HX discharge temperature, what you need to figure out is what amount of time the tank is at average temperature below a deadband setback, maybe 95.5*. Don't think the answer can be given from the info you provided.

 

[Kuhuh]

The original idea is to show the energy savings of preheating the water with the heated water from the PV cell. I didn't clearly think it all the way through as "IRstuff" pointed out. There are two points I severely missed.
1. Most water heaters are kept at 120^oF if the effluent water from the PV cell is only 98^oF then the heat exchanger system would be cooling the water heater rather then heating it.
2. Water heaters are usually kept running at the set temperature therefore it would be hardly ever be below 98^oF so the system would be pointless.

My solution is to
1. To heat up a tank of water to 98^oF then add individual instant heat systems. Such as some dish washers have. My goal is to obviously prove how to reduce electrical use by means of heated water.

Any suggestions on new idea? I have no experience with instant heat systems.

 

[IRstuff]

?? Instant heat as in tankless? Those are all electrically or gas driven.

If your heat exchanger water is potable, you could ostensibly substitute the 98°F water for the 68°F inlet water with some sort of thermal shunting valve system, or if not, you'd run a counterflow exchange with the inlet 68°F water. In either case, you'd preheat the water before it gets to the actual water heater. Note, however, the latter approach would only raise the temperature about half of what direct substitution would get you.

120°F setpoint: it's actually worse than that. A typical water heater probably heats up to ~140°F during the heating cycle, and then idles and cools down to probably below 120°F before the next heating cycle kicks in.



TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Kuhuh]

If I were to say directly used the 98F water what kind of pricing comparison do you think I'd see between the normal water heater application. And the tankless water heater

Im thinking the Tankless heater option would need a sense line probably 10 pipe diameters ahead to indicate temp. of water coming in and turn on the heater coil. Does this cause a hesitation in the disbursement of the heated water more so then from a water heater. Do you think the annual maintenance cost (or total ownership maintenance) more then a standard application.

 

[IRstuff]

Ostensibly, you'd save more than half of the heating cost, right? 120°F - 68°F = 52°F vs 120°F - 98°F = 22°F, at least, for the amount of 98°F water you have available.

Since there are storage losses, these also add to the cost, i.e., every time the water heater goes into a heating cycle when no water is being used.

A tankless system will not have that loss, since it's an on-demand heater. There will be a lag, since these systems monitor flow and require a certain level of flow before they start heating. Some people find that objectionable, but others do not.

A tankless system dumps a substantially higher amount of power than a regular water heater. This is most noticeable in the exhaust requirements, which is usually 2 short stacks that must immediately exit the garage, or whereever it's installed, compared to the typical water heater B-vent that exits from the roofline. The gas inlet is substantially larger, I think. Noritz

http://www.noritz.com/homeowners/products/residential_use/

shows up to 250kBTUh, which is about the equivalent of 15 of your largest stove burners all running at the same time.

Here's a discussion about economics of water heaters:

http://www.consumerreports.org/cro/...eaters/overview/tankless-water-heaters-ov.htm

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize