A Btu meter simply measures the amount of heat a solar collector is adding to a fluid at a given point in time. A Btu data logger, on the other hand, measures and records the amount of heat added over an extended period of time.
By taking measurements at different times of the day, under various weather conditions, a reasonable estimate of solar collector performance can be determined.
If you desire a more accurate picture of how your solar collector performs, with data you can analyze, a Btu data logger should be considered. However, it will cost significantly more then the simple BTU meter we present here.
The information we seek with a simple Btu is the temperature of the water entering the collector, the temperature of the water leaving the collector and the flow rate of water through the collector. Our goal is to fill in the blanks in the following equation:
Flow Rate in Lbs/Hr. is converted from gallons per minute (GPM) as follows:
The DLJ meter is available with 1/2" or 3/4" fittings while the Minol meter is only available in 3/4". Simple adapters can be used to fit it to 1/2" or 1" pipes. Although the basic DLJ meter is less expensive than the Minol meter, optional upgrades for the DLJ meter bring its final price in line with the Minol unit.
If you are considering a future upgrade to a Btu logger, plan to buy a flow meter with a pulse output. The pulse output is standard on the Minol Meter but a $20 option on the DLJ meter.
It is best to locate the flow meter on the output (hot) side of the solar collector so any reduction in flow due to pressure drop in the collector tubes will be taken into account. The hot water feature is another $20 option on the DLJ meter but comes standard on the Minol meter.
The most economical way to capture temperature is to buy a couple of indoor/outdoor digital temperature gauges at your local home center or you can order them on line. Be sure the gauges have a wired remote sensor to collect outside air temperature. Prices are typically $10 to $15 each.
The remote temperature sensor is strapped directly to the side of a 4" to 6" copper or brass nipple using a hose clamp, tie wrap or black electrical tape. Since copper is an excellent conductor of heat, a probe into the fluid is not required. You may want to add a piece of pipe insulation around the assembly for more accurate readings.
This nipple assembly is spliced into the PVC plumbing on either side of the solar collector. Be sure to use female metal fittings when mating with PVC threads to prevent splitting. If you want to account for the heat loss of the piping between your solar collector and the point of use, locate the nipples close to your hot water heater.
If the temperature gauges can be located in a dry area near the solar collector, the indoor temperature reading can be used to give an ambient outside air temperature (OAT) while the outside sensor connects to one of the pipe nipple assemblies. Tracking OAT data will give you a better idea of how your solar collector performs in different weather conditions.
Locate the flow meter and each of the temperature sensors next to each other so you can take readings from all of them at the same time. The flow meter will need to be mounted horizontally, face-up, to operate properly. If your temperature sensors are located outside, build a small enclosure that keeps them dry but does not insulate them from the ambient air.
Using the Btu Meter
Build a table for collecting Btu meter data with the following information:
Record date, time and temperature readings. Monitor the flow meter closely counting the number of revolutions the small red indicator makes in a minute. Each revolution corresponds to one tenth of a gallon.
Take three or four readings during each session to ensure the Btu meter is reading consistently. If you record data each hour the solar collector is active, you will get a more realistic picture of total daily Btu output. Record data on sunny days, cloudy days, warm days and cool days noting weather conditions in the remarks column.
Transfer your figures to a spreadsheet and use the equations above to calculate Btu output. Graph the output measured by day and compare performance across different weather conditions. The area under the curve of the graph will be energy in Btu output per day.
Average the daily readings using a pattern that reflects your typical weather conditions. In other words, if you typically have 3 cloudy days and 4 sunny days per week, prorate accordingly. Depending upon where you live, this will vary between wet and dry seasons of the year.
Multiply the average daily Btu's times the number of days in the month. Knowing the monthly Btu output of your solar collector you can determine equivalent savings on your electric bill through the following conversion:
or your gas bill:
Simply multiply the converted kilowatt-hours or therms by the current rate on your utility bill to find out your monthly savings. Divide this into the cost of your system, including the cost of the Btu meter, and you will know how many months it will take to recover your investment.