Subject: How to control solar collector inlet temperature?

From: [email-masked] (Petri Konttinen)
Date: 09 May 1996 14:54:14 +0300

Hi there!

I have a practical problem:

How to regulate inlet fluid temperature for a solar water heater so, that
it will remain within +- 1 Celcius from the temperature wanted?

We have a outdoor test facility for solar collectors, and we want to test
different sizes and models of collectors. We want to input (mainly) water
into a collector in different temperatures, e.g., beginning from ambient
temperature (20 C), feeding the collector for, say, half an hour, then
during a few mimutes change the temperature to 35 C, keep it there for
another 30 mins, change it to 50 C, etc.

At the moment we have a 3-way valve with regulator controlling the inlet 
temperature. One pipeline feeds it with cool water, and the other has an
electric by-pass heater. Now the problem is, that the heater cannot keep the
water temperature fixed, but it fluctuates within a range of +-10 degrees.

I've planned to remove the heater and to connect the other pipeline coming
directly from the collector outlet. The other pipeline would still input
cooled water. The whole system is closed, and includes a 200-litre tank with
expansion-unit.

->      pump				     ->	_________
--------(->-------------------------------------|	|----<
-- solar		-			|	| cooling
-- collector		- 	  		| tank	| circuit
--          3-way valve	- B-inlet (down)     	|	|
------------------------|-----------------------|_______|---->
<-           AB (left)    A-inlet (right)    <-

Now, the problem will be (?), that the outlet water temperature from the 
collector will not remain constant. This will set high demands for the valve
and its controller to cope with the changing temperatures.

Does anyone know of any similar test facility?
If so, does it work?

Can you give me hints if there is any other decent way to control the 
collector inlet temperature in a closed circuit? 

Flowspeed will remain constant during the test, although we want to test
it with different flow rates, each separately from others. The minimum
flow rate will be approx. 50 litres/hour and the maximum 600 litres/hour.

Thank you for any advice/pointers!

Petri Konttinen