|There are a lot of recommendations around on what the flow rate
through water heating solar collectors should be.
I'm aware of these:
So, what is the "best" number, and what are the tradeoffs?
Skip to the bottom line...
The main thing you want is a flow rate that efficiently removes the heat that the sun is depositing in the collector.
Basically, flow rates that are too low will not remove the heat efficiently from the collector, and the efficiency of the system will be low. Flow rates that are too high will require larger pumps and plumbing that increase both initial and operating costs. So, you want just the right flow rate.
The heat into the collector is the Solar Intensity times the efficiency of the collector. So, for good sun, and with a typical flat plate collector efficiency of 50%, the solar energy in is:
QsolarIn = SolarIntensity * Eficiency = (300 BTU/hr-sf) * (0.5) = 150 BTU/hr per sqft of collector.
So, for typical full sun conditions, 150 BTU of energy need to be removed from the collector for each sqft of collector.
The heat that the fluid flowing through the collector will remove is:
Qout = Trise * FlowRate * SpecificHeat
Where Trise is the temperature rise in the fluid as it passthrough the collector. Trise = Toutlet - Tinlet, where Toutlet is the outlet temp of the collector, and Tin is the inlet temperature (tank temperature).
Flow Rate is the fluid flow rate in lbs/hr.
SpecificHeat is the specific heat of the fluid -- that is, how many BTU are required to heat 1 lb of the fluid 1 deg F (1.0 for water)
When the collector is operating in a steady mode, the QsolarIn must be equal to the Qout -- that is the energy into the collector must be equal to the energy being removed from the collector.
What is the effect of change the flow rate?
If the flow rate is quite low, then Trise must be high in order to remove the solar heat.
This means that the average temperature of the absorber is high.
This is bad because the hot absorber loses more heat through the collector glazing, and this reduces the efficiency of the collector.
High flow rates result in a low Tout - Tin, and this is good for efficiency. But, as flow rate is increased, eventually diminishing returns set in, and the gain in efficiency for further increases in flow rate are small. When you couple this with the fact that high flow rates require larger pumps and larger pipes there is a point you do not want to go beyond.
What this boils down to is that to remove the heat from the collector you can heat up a small flow of water a lot, or a larger flow of water a little. The collector will operate more efficiently if you heat a larger flow of water a little.
The table below gives shows the drop in collector efficiency and in heat output as the flow rate is decreased.
The values in the table are based on these conditions:
- The collector efficiency curve is equivalent to a typical commercial flat plate collector with black painted absorber.
- The inlet temperature is 100F, and the ambient air temperature is 40F
- Full sun conditions.
- The flow distribution inside the collector is uniform (that is, all risers get the same flow rate)
- The heat transfer fluid is water
Drop in Heat Out
The last column gives you the drop in collector output you will suffer as you choose lower and lower flow rates from a high flow rate base of 0.07 gpm/sf.
For example a flow rate of 0.03 gpm/sf will cost you a 3.5% in the heat output of the collector compared to a high flow rate of 0.07 gpm/sf.
You can look at the flow rates, and see where you want to come in.
Flow rates at the high end will make for a thermally efficient collector, but may require a larger pump and larger pipes.
Flow rates at the low end may be attainable with a smaller pump and lower diameter pipe, but will be less thermally efficient.
For systems I look at, I start by seeing what kind of pump and plumbing would be required to attain a flow rate of 0.04 to 0.05 gpm/sf. These are flow rates that will result in a thermally efficient system.
If the pump and/or plumbing size to attain the 0.04 to 0.05 gpm/sf seem depressingly large, I would see what dropping down to 0.025 to 0.035 gains in terms of a smaller pump and plumbing, and consider doing this if the gain is large enough.
A further advantage of shooting for a relatively high flow rate (0.04 to 0.05) is that if you underestimate the pressure drops through the system, then while the flow rate will be lower than your estimate, it will not be so low as to cause poor performance.
If the system is using glycol, the specific heat will be reduced in proportion to the fraction of antifreeze. So, a higher flow rate is required to remove the heat from the collector. Glycol also increases the fluid viscosity, so friction losses are larger, and a larger pump is required both for the greater friction losses and the high flow rate requirement.
There are other factors that may influence flow rate. For example high flow rates that result in pipe velocities over 5 ft/sec may cause erosion problems, and low flow rates may result in problems getting the system started for drain back systems.
This page provides a procedure for sizing the pump and plumbing for a system ...