September 22, 2011
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I am back from a bit of a break and have been doing some solar monitoring projects using flow meters. The benefit of the flow meter is it allows you to do an energy calculation and determine just how much energy the solar panels are saving you. Up until now I have been working on measuring the glycol energy, and the glycol uses a constant flow rate. The water side of the system is different because the flow is based on the buildings hot water demand. Without a flow meter or an estimated flow rate all you are really doing is measuring temperatures. Flow meters work by sending an electric pulse every time a specific volume of fluid passes them. Most give one pulse per gallon or one pulse per 10 gallons. The Web Energy Logger (WEL) counts how many of these pulses you receive each minute. You can easily scale this value with the WEL’s programming to whatever engineering units you want. I prefer liters per second. Once you have the WEL measuring the flow properly you can move on to the energy calculations.
The formula for calculating the solar output is as follows:
Solar Heat Output = (volume flow rate)*(density of the fluid)*(specific heat of fluid)*(temperature differential created by the solar system)
You can also calculate the total heating required by the building by measuring the temperature differential between the cold water supply and the final hot water on the outlet of the furnace. You apply the same formula as the solar output but using the new temperature differential.
Total Heating = (volume flow rate)*(density of the fluid)*(specific heat of fluid)*(temperature differential created by the entire heating system)
The total heating value can be used to calculate the percentage of the total water heating load that the solar system is able to meet. This percentage is known as the solar fraction.
Solar Fraction = (solar heat output) / (total heating) * 100
If you are monitoring an active solar system (one that uses electricity to run) then you will need to subtract the electrical energy used by the pump and control system from the solar output to get a fair representation of how much you are indeed saving. In most cases the equipment will use a minimal amount of energy but it is important to factor it in because over the course of the year it does add up.
Once you have the WEL calculating these values you can start logging daily, monthly, and annual totals. This will provide great insight into the effectiveness of your solar thermal system. It is important to note that without measurement you can never be sure that the system is working even if it is brand new. The math formulas and calculations for monitoring solar systems might seem a little tedious at first but before long it will become old hat.
An example of this in action can be found here: http://www.welserver.com/WEL0512/
A flow meter with an electronic pulse head counter
The above flow meter installed on a domestic hot water system