Calculating total heat transfer by solar water heater

[modernhistorian]

TL;DR

Given a tank of known volume and knowing the temperatures of water as it enters, remains, and leaves the tank, is it possible to calculate heat gained or lost?

I have a solar water heater on my deck, with a circulator powered by PV, that pumps heated glycol through a wand heat exchanger placed in an 80 gallon tank. Water comes from the well's pressure tank, passes through this solar pre-heater tank, then into an LPG heater that adds any heat needed. I want to calculate, in my home automation software, the amount of heat in B.T.U's that are produced in the water flowing through this system. I can measure:

1. The temperature of the water as it enters and exits the system, and in the tank. As well, the temperature of the heat exchanger.
2. The flow of water through the system, which is of course intermittent.

if I can make this calculation periodically, then the software can total it. any guidance would be appreciated.

 

[russ_watters]

If you have the temperature and flow, a BTU is the energy required to heat a pound of water by 1 degree F.

...depending on the glycol fraction it may be significant enough to correct for its heat capacity difference (it is less than water).

 

[BvU]

Hello @modernhistorian ,

$\qquad$ !​

I have to pass on the units you want to use -- so conversion is left up to you.

You may want to make a diagram of your setup (like here), indicating measurement points, so you can name the variables that occur in the calculations.

Heat flow $\dot Q$ ( in my SI units J/s or W ) is $$ \dot Q = \rho \, c_p \, T\,\Phi$$ with $\rho$ density in kg/m³ , $c_p$ heat capacity in J/(kg$\cdot$K), $T$ the temperature, and $\Phi$ volume flow in m³/s .

With this you can set up a heat balance. (detailed example here)

From your description I understand that you have $\dot Q$ for water at various points. What about the glycol ?
(i.e. I don't understand 'the' temperature of the heat exchanger -- there should be four temperatures: water in and out, glycol in and out)

$\$