4.1. Thermal energy
Thermal energy is the total internal energy of the atoms or molecules of a substance. Heat is thermal energy that is being transferred between two places and is measured in the same units as energy.
However, there are units that are commonly used for thermal energy. In the British system of units, thermal energy is normally measured in British Thermal Units (BTU).
In scientific work, the usual unit of thermal energy is joule and the calorie (c) or the kilocalorie (C), which corresponds to 1000 calories. One calorie equals 4.19 joules.
Thermal energy is also measured in British Thermal Units (BTU): 1 BTU equals 253 calories.
The transfer of energy as heat can take place via three processes: conduction, convection, and radiation.
In conduction and radiation (infrared radiation), energy transfer occurs without mass transfer. In many cases, we wish to prevent heat transfer. For example, the less heat leaves our home in the winter, the less energy (and money) we need to keep it comfortable.
In case of radiation transfer, all the laws and optical rules can be applied in the description of phenomenon. When we are trying to minimize thermal energy transfer, we must take all three kinds of heat transfer into account.
We often wish to minimize the amount of heat transferred between the inside and outside walls of homes and buildings. If we assume that we cannot change the outside temperature and we are not willing to reduce the wall area, we must make the wall a very poor heat conductor while making it as thick as we can.
(3.7)
Where:
K = the thermal conductivity (J/s m °C or BTU inch/hour foot2 °F)
A = the cross sectional area (square meters or square feet)
T = temperature (°C or °F)
L = thickness (meters or inches)
The R-value of insulation is the ratio of the wall thickness to the thermal conductivity K of the material. The larger the R-value (thermal resistance), the better the insulating property of the building material.
(3.8)
Where:
R = the R-value of the material
4.2. The thermal energy meter
A thermal energy meter (or BTU meter) is able to measuring heat flows through devices such as hot water heaters, heat exchangers, waste-heat recovery units, or solar heating systems.
Where heat is taken from a fluid, the energy meter can measure the amount of heat exchanged. Heat is measured by connecting temperature sensors to the fluid inlet and exit pipes, and by inserting a volumetric fluid flow sensor in either the inlet or the exit pipe.
The meter measures the fluid temperature difference between the inlet and the exit and multiplies it by the fluid flow rate and a thermodynamic coefficient. The result is the energy flow rate. This rate is accumulated as the total heat flow.
Figure 3.12 Block diagram of a water heater energy monitoring system
Figure 3.13 Block diagram of solar heating system
A thermal heat meter is not complete without temperature sensors. Calibrated and approved as an integral part of the unit, temperature sensors can fit in pockets, ball valves or directly in the liquid flow. Usually the meters are integrated in a system. The system will be based on standard components, such as meters, and hardware and software for data reading.
4.3. Methods employed in thermal energy measurements
a). Ultrasonic technology – transit time method. Measurements are made by means of bursts of signals sent through a pipe. The measurement of flow is based on the following principle: traveling in the fluid flow direction, sound waves require less time instead of traveling the opposite way.
At zero velocity, the transit time or Dt= t21 -t12, is zero. If we know the pipe’s diameter, the distance between the two transducers, the pipe’s wall thickness and the pipe’s wall material, the angle of reflection can be calculated and we will know how far apart we should space our transducers.
Animation 6 Block diagram transit time method meters
Figure 3.14 Transit time diagram: T1,T2 ultrasound transducers ,t12, t21 transit time T1->T2, and T2->T1
Ignoring the v2 value, which is extremely small compared to the c2, we obtain:
The difference in transit times for the ultrasonic signals is an indication of the fluid flow rate (F). Since ultrasonic signals can also penetrate solid materials, the transducers can be mounted on the outside of the pipe. Fast Digital Signal Processors and signal analysis guarantee reliable measuring results. This is true even under difficult conditions, where ultrasonic flow meters have previously failed.
b). Doppler flow meter method:
Flow meters operate on the Doppler shift effect, whereby the transmitted frequency is linearly modified due to its reflection by particles and bubbles in the fluid. The net result is a frequency shift between transmitter and receiver frequencies that can be directly related to the flow velocity.
(3.15)
Where f0 is the base frequency; v is the velocity of fluid flow; c is the speed of ultrasound in the fluid and j represents the incidence angle between ultrasound direction and fluid flow.
Animation 7 Block diagram of Doppler flow meters
If the pipe internal diameter is known, the volumetric flow (F) rate can be calculated.
(3.16)
Doppler meters require a minimum amount of solid particles or air in the line to achieve measurements.
4.4. Communication module
The heat meter communicates with the utility, via the communication module. On the one hand this converts the measured values in messages that are transferred, using wires or wireless transmission channels; on the other hand adds utile information, like check codes and other information about source and target of messages.
The communication protocol consists of rules assemblies (electrical parameters and logical structure) that manage the transfer of data form. A simple hand terminal can be used if manual remote reading is required. The communication protocol can be one of the following:
- BMS
- RS 232
- Modem
- Radio
- M-Bus
- Tap meter inputs changes
- Energy pulse output
See the chapter referring to the protocols used for a house.
4.5. Goals of automatic meter reading 401
The prime objective of an automatic meter reading is to ensure that customers are accurately billed for their energy consumption, as quickly and efficiently as possible, by remotely reading consumption data. Data can be transferred to the billing system and to analyzing programs. Alternatively, the process can be totally automatic, so that the utility can focus on plant performance, customer service, and invoicing customers for energy consumed.
Load patterns can be a major factor for effective savings both for the utility and the customer. Encrypted and secure transfer of consumption data can be published on the Internet and accessed by the individual customer via a personalized logon and password.