Water heating

[copey84]

Hi all, was looking some advice on using a solar panel to heat water through summer months.
Instead of a large PV arrangement I was hoping to buy a single panel that could output max 250w, and connect it directly to water heater via change over switch.
My immersion heater is rated at 2500w and takes around 30mins to heat water, if It was feed at only 10% which is the 250w solar power then would it be capable of heating the water in 5 hours?
I calculate that it will take 10 times longer at 250w, so 30minx10=300/60= 5hours, could this be possible??
Also the sole purpose of panel is only to heat water, so no need for an inverter.
Appreciate any replys.

 

[PEG]

Hi,your panels' output would be best case,and still,it is short by a factor of ten. What is it you hope to achieve? :flowers:

 

[Martin Kerr]

How are you proposing changing the output from DC to AC?

 

[davesparks]

How are you proposing changing the output from DC to AC?

Why would they need to?

 

[Deleted member 9648]

I suspect you would need to take into account heat loss from the storage tank before concluding it would work.A 250w input over many hours may well struggle to increase water temperature at a rate exceeding heat lost from the tank over that period of time.

 

[johnboy6083]

So, E = (mC Dt)/t

m = mass in kg
C = 4.187 for water @ 10 degrees C
Dt = difference between input and output temp. I expect it will be around 55 degrees
t = time in secs

So for a 180 litre tank, to heat up with 250W will take around 46 hours to heat up (assuming no heat loss)
Far too long, you really need around a 2 HR reheat time according to NHBC guide and CIBSE guide G.

 

[telectrix]

could always accelerate the warming process witrh a bunsen burner.

 

[copey84]

Could be waiting a bit for hot water in summer then, if 46hours is right.
Ah well it was just an idea I had , wanted to generate my own electric and thought heating hot water from solar panels would have been a good project.
Thanks for all replys

 

[davesparks]

Better to have a wet solar system thingy that heats the hot water

 

[Lucien Nunes]

There is a problem with connecting panel output directly to a heating element, due to the output curves of the panel. With decreasing irradiance the available current at a given voltage falls much faster than the voltage, i.e. the effective dynamic source resistance increases. Since the resistance of the heating element remains constant, you cannot utilise the maximum available power at more than one level of irradiance.

Put another way, if you configure it to maximise output on a dull day, it won't do much better on a bright one, or if set for max output on a bright day the performance will plummet on a dull one. One of the many functions of an inverter is to sort this out and present an optimum load to the panel to suit the prevailing conditions.