# Calculating the output temperature of a Solar Thermal system

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## Main Question or Discussion Point

So say I have a PV-Thermal array and assuming standard test conditions.
How would I calculate the temperature of the water coming out of the system?

The thermal output of the system is 67.5kWp with a maximum flow rate of 65L/h. Approximately 500m of 3/4" pipes.

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Andrew Mason
Homework Helper
If you know the specific heat of the fluid substance in the pipes and the initial temperature, use: $\dot{Q}t = mc\Delta T$ where $\dot{Q}$ is the rate of heat flow out of the fluid (67.5kW), t is the time during which heat flow occurs (one hour), m is the mass of the fluid from which the heat flow occurs (the mass of 65 L of fluid) , and c is the specific heat of the fluid (heat flow/unit mass).

AM

Khashishi
What is a kWp? Is that a typo?

Andrew Mason
Homework Helper
What is a kWp? Is that a typo?
Not a typo. It is a term used for solar panels to denote the power output under optimal conditions with maximum solar radiation.

AM

Is there way to take into account a continuous input of 20 degrees Celsius water at the given flow rate?

Andrew Mason
Homework Helper
Is there way to take into account a continuous input of 20 degrees Celsius water at the given flow rate?
$\dot{Q}t = mc(T-T_i)$ where $T_i$ = 20C. Work out from the pipe volume the volume of the fluid in the pipe. Then divide by the flow rate to determine the length of time (t) that the fluid will be absorbing heat flow at the stated rate. According to my calculations, fluid will be in the pipe for a bit more than 2 hours. During that time, figure out how much heat a unit of fluid absorbs. From that you can work out the temperature difference.

AM

Last edited:
I understand the equation, but it doesn't take into account the constant flow of water in. After doing the math with the equation you provided I get delta T to be 235 degrees celsius which seems like a ridiculous value.

Andrew Mason