By Rick Ellul
Thermal energy meters (TEMs) are becoming more and more important in buildings, particularly in smart buildings. LEED, BOMA Best and other building energy efficiency programs, along with standards such as ASHRAE 90.1, have brought to light the importance of knowing how much energy we are consuming and where we are consuming it. TEMs, which are also known as BTU meters or heat meters, measure the thermal energy in a fluid. In the case of the HVAC industry, the fluid for the most part is water. The devices allow us to target with great accuracy where energy is being consumed and at what rate.
We can’t know how buildings are performing unless we measure. We have been measuring electricity and gas consumption for years. It’s only more recently that we have started to utilize TEMs in North America to inform us what is happening with the energy in the fluids we move around buildings to heat or cool them.
Thermal energy meters are found in all types of buildings, including ICI and multi-unit residential buildings. We also find them in some production facilities such as commercial growing operations and beer production.
What is a thermal energy meter?
How many of you recall being in trade school, college or university and studying thermodynamics? Remember how much fun thermodynamics was? I bet you do and yet you are still here in the industry!
Do remember the formula for calculating energy for a liquid?
500 is a constant for water that allows us to get a rate of change i.e. BTU/H. It represents one gallon of water at 8.33 lbs x 60 minutes in an hour. The GPM is self explanatory as gallons per minute of the fluid.
The DT is the temperature difference of the fluid. With this formula we can figure out how much energy is moving through a heat transfer device, such as a coil or heat exchanger, in BTU/H.
A TEM measures flow of a fluid and has two temperature sensors to measure supply and return temperature so we can determine the temperature difference. It also needs a calculation to take the flow and delta T and turn it into a measurement of energy.
How are TEMs Used?
There are three main uses for TEMs today that will grow exponentially in the coming years. The first is energy monitoring and energy control. In this scenario the energy is measured continuously at the point of use and the data from the device is used either by the building’s management system, or a third-party analytics tool, to make decisions on how to control the energy usage in a specific area or areas of a building.
As an example, TEMs installed on all of the air handling units in a hospital would be used to measure the energy used at the chilled water coils or heating coils. This could be done with a standalone meter or a combination TEM and control valve.
In the case of a combination TEM and control valve there are extra benefits of having one device that can measure and record the thermal energy. In addition, the combination may offer extra functionality to optimize the energy transfer, provide fault detection and diagnostics of the device, receive updates from the Cloud, and also communicate with the building’s BMS system.
The second scenario is allocation of energy costs to tenants. For example, in an office tower with a central chiller system we want to know how much the tenant on the sixth floor is using every month so they can be billed for what they really use. This could also be done with a standalone TEM, or with a combination TEM and control valve. This type of application provides transparency to both the tenant and building owner.
The third application is direct billing of energy costs to tenants. For example, a new condo in downtown Halifax, NS, has a meter on the fan coil in each suite. The meter gathers the data on the usage of thermal energy in that suite and it shares the data directly out to the Cloud where, on a monthly basis, the total usage is aggregated and then billed directly to the tenant either by the property management company or the metering service provider.
Types of meter measurement
TEMs must measure flow as part of their way to calculate the energy. There are several ways to measure the flow of water. The more common types are turbine, vortex, magnetic and ultrasonic.
The turbine meter has a propeller or paddle wheel that spins and counts as it rotates. The counted rotations measure volume. The vortex meter uses an obstruction in the flow to create a swirl or vortex that a flexible sensor can sense as an oscillating frequency, thus creating an output that can be used to calculate the flow.
Magnetic flow meters utilize a magnetic field, which measures the speed of the fluid passing through a pipe. As the fluid passes through the magnetic field a voltage is created. If the velocity increases so does the voltage. The voltage is converted into a flow volume that is used by the meter to show the flow measurement.
An ultrasonic meter measures the sound travelling between two transmitters. One pulse is measured downstream with the flow and the other upstream against the flow. The difference in transit time is used to calculate velocity and the meter then uses that velocity to calculate volume. Since it is sound that is travelling the condition of the water is irrelevant.
These meter types can measure the volume of water, but with varying degrees of accuracy and longevity. Note that only magnetic meters and some ultrasonic meters can measure water with glycol.
Part II of this article in an upcoming issue will discuss metering standards and regulations, and how TEMs share data.