Published: 20 January 2010
Guessing is Not an Option
Before detailed design can be done, many important planning steps have to be made:
Collecting basic information Rough dimensioning Cost estimate Simulation Design optimization
and only then...
Collecting Basic Information
Let us start today by talking about the first step: Initial object data input - or Collecting Basic Information.
You see - because of varying energy supply from the sun, we need to know precisely the volume of hot water consumption, the consumption profile and desired water temperature. By having fewer variables in equation the result could be more accurate. To have that information is critical because all further dimensioning tasks are based on it.
The values for DHW consumption data given in different literature have been found too high in practice; and result of their use is over-sizing of the solar system with all other consequences. Often these references are old, and have been calculated to cover highest possible demand factor.
Imagine - you are just breaking-even finishing the job and the customer is demanding to fix the overheating issues on warranty... - nice prospect, isn't it?
Another very important sizing criterion is the consumption profile.
Similar buildings/institutions can have absolutely different load profiles. For example: 100 room student dorm of the Faculty of Music and 100 room dorm of the department of Physical Education. In the second one the students are taking showers after the gym, outside the dorm.
Once again - the consumption volume and profile are so crucial in getting your design done right that you don't need to even think of "guesstimating" it, but measure it!
Getting Accurate Measurement Data
Placing the water (BTU) meters to measure daily, weekly demand, seasonal/occupational profile is the only way to get an accurate data from existing buildings.
- The following parameters should be measured:
- Volume flow through the water heater
- Hot water supply temperature (tank outlet)
- Cold water inlet temperature
- Recirculation losses (especially if you intend to cover these losses with solar)
- Number of residents
The correct location of water meters and temperature sensors are crucial for the accuracy of the measurement.
The hot water meter has to be positioned at the cold water inlet of existing water heater tank. Make sure that there are no any cold water branches or additional flow between the meter and tank. One exception could be the recirculation return feeding not directly into the DHW tank but upstream of the tank into the cold water pipe instead. The volume of recirculation should be excluded from the measurement.
The meter should not be installed in the hot water outlet of the tank because it will measure the recirculation flow at this point. This location is only acceptable if recirculation volume is measured with separate meter and will be deducted latter on. All meters placed in correct locations could be used in a future for monitoring of solar system performance.
Choosing the right meter model is important for accuracy of your measurements. The nominal flow of the meter has to be carefully estimated. Experience shows that water pipes are often oversized, while using water meters with same connections size, results in higher minimum flow range which could lead to inaccuracy especially by small flows. A water meter has to be chosen one trade pipe size smaller than the pipe it is installed on.
The measurement period must not be less than 6 weeks. The total volume measured has to be divided by the number of the measurement days in order to obtain the mean consumption value.
To get the real picture of consumption profile daily if not the hourly readings are required. By having that information available some pretty interesting conclusions could be made.
First, all abnormal days could be recognized and eliminated out of your calculation; secondly different profiles could be concluded for working days, weekends and public holidays. That could to be used as input for simulation programs. And don't forget this data will be an eye opener for most of your customers in terms of allowing them to recognize an additional savings potential!