Reference 1 Thermal Energy Metering Project Reference: Metering_Thermal Industry or sector: All Energy use: All Date added: Sep 2011 Thermal Energy Metering Abstract: Some aspects of the selection and installation requirements for thermal energy metering are canvassed. Situation While thermal energy metering is common in colder European climates (think district heating schemes), it is only now becoming more common in Australia, primarily on the back of performance monitoring such as for GreenStar, NABERS, co-generation, tri-generation and the like, as well as for accurate and equitable energy and cost allocation to end-users for energy derived from a centralised source. Large industrial grade thermal energy metering is well understood, but there are more costeffective systems for non-industrial applications such as in smaller industrial, other commercial and larger residential situations. The lack of a long-term history of thermal energy metering in Australia in medium size situations means that it is difficult to obtain useful information on available products, support may be hard to find and selection criteria may be uncertain. This document is an attempt to go some way to overcoming these obstacles. Basics For the purposes of this document, thermal energy is the delivery of heat energy (heating) or removal of heat energy (cooling) via a circulating liquid, usually water. While other scenarios are possible, they are not considered here. The instantaneous thermal power [P] delivered or removed is determined by the following formula: Where: P = m c p (t out - ti n) or P = V c p (t out - ti n) m is the mass flow rate of fluid (kg/s), V is volume flow rate of fluid (m 3 /s) is fluid density (kg/m 3 ), taken as 1000 for water over the expected working range c p is specific heat of the fluid, assumed constant over the working temperature range (kj/kg.k) t in is the temperature of the fluid supplied ( o C) t out is the temperature of the fluid returning ( o C) t is the temperature difference (or t out t in), ( o C or K, same value). If positive, the fluid is removing heat (cooling) and if negative is delivering heat (heating) P is thermal power, units are kj/s or kw
Reference 2 Thermal Energy Metering Instantaneous power is often of little use and may sometimes be misleading due to the time lag between fluid passing from the t in measuring location and the t out measuring location. Mostly, the need is to measure thermal energy which is thermal power integrated over time. In this way, small error in instantaneous thermal power readings are minimised. A thermal energy meter (TEM) is the device which allows measurement of transferred thermal energy and is considered to comprise four (4) components, some of which may be integrated: Flow measurement component (to measure m indirectly as the product of volume flow rate and fluid density) Temperature measuring sensors, two (2) required for fluid sent out to (t out) and fluid coming back (t in) Calculator (electronic box) to integrate 1 thermal power (P) over time (T) being the product of mass flow and temperature difference (effectively energy, E = P. dt ) or more commonly, volume flow and temperature difference with a constant applied for density. Selection Requirements The basic questions that need to be answered are: What is the expected maximum flow rate to be measured? What is the expected minimum flow rate to be measured? (Turndown ratio is a common which is the maximum flow rate divided by minimum flow rate) What flow measurement accuracy is required? What is the expected maximum temperature to be measured? What is the expected minimum temperature to be measured? What temperature measurement accuracy is required? What overall system measurement accuracy is required? Supplementary questions relating to an overall system in which the TEM may be found which may need to be answered are: What data connectivity is required? Such as to building management or other system, webbrowser, and so on. Or none, meaning manual read type. Is data storage required? Local, remote, none? Are there special data storage requirements? Such as end of month values. Are there special restrictions on installation of flow measurement component? May include only in horizontal, or vertical with flow upwards, or condition of fluid such as cleanliness, conductivity, entrained gases, unimpeded straight length upstream and downstream and the like. Does the installation location cause the need for special protection such as from weather and the like? Is the equipment accessible? 1 This would be true if the temperature and flow signals were continuous in nature. However, for most systems, the volume (mass) flow signal is in the form of discrete pulses so the data is captured in a pseudo-digital way (like a step function?) which may assume fixed conditions of temperature over the period since the previous volume pulse.
Reference 3 Thermal Energy Metering What special requirements are there for temperature sensor insertion, such as wells or pockets? Are there special installation requirements such as proximity to bends, need for insulation protection and the like? Is power required to be supplied? Is the device mains-direct, or low-voltage (AC or DC)? Is there an internal battery for memory? Is the device bus-powered? If connected on a data bus, what special requirements are there for cabling for high speed data transfer, minimisation of interference, drop-outs and the like? Wireless option? What measurement units are required (kwh, MWh. kj, MJ, GJ) and what discrimination (1 unit, 0.1 units, 0.01 units, etc.)? Does the calculator require proprietary software to be set up, tested and communicated with? Is it user-friendly and up to date? Is training and support available? What local product knowledge and support is available? Can a complete package be supplied? Does the supplier offer a programming/commissioning/troubleshooting service? Would pulse input capability be useful such as to collect data from other meters nearby with basic pulse outputs? This could include water meters (cold/hot) and gas. Are there restrictions in terms of flow resistance that can be allowed arising from the added flow measurement component? This may lead to a larger device being selected to reduce added resistance. It should be apparent that most of these supplementary questions relate to the capabilities of the calculator component. Example A plant is supplying chilled water for a building with a flow rate, of 25 L/s (m say 25 kg/s) with supply temperature (t in) of 7 o C and a return temperature (t out)of 13 o C (t of 6 o C). P = m c p (t out - ti n) P = 25 x 4.19 x (13 7) = 628.5 kw The positive value signifies removal of heating power from the building or cooling. If this cooling power remained fixed over an hour, the energy removed would be 628.5 kwh or 2262.6 MJ (1 kwh is equivalent to 3.6 MJ). Assuming this is the upper limit for chilled water flow rate, the flow measurement component is selected to suit. Typically such devices have a nominal continuous flow rating (variously referred to as Q n or nominal flow rate and q p or permanent, continuous flow rate) in cubic metres per hour (m 3 /h). The flow rate is 90 m 3 /h, so initial selection might be a device with a Q p of 100 m 3 /h. This device will probably have a volume pulse rate of 1 pulse for every 100 litres (0.1 m 3 ) of water passed. So at the expected flow rate of 25 L/s, a pulse would be generated every 4 seconds.
Reference 4 Thermal Energy Metering Flow Measurement Some aspects of terminology are introduced here as there does not yet seem to be a common one in use. The normal full flow rating is variously referred to as Q n or nominal flow rate and Q p or q p for permanent, continuous flow rate The maximum allowable flow rate is usually two times Q n and has been seen as Q s (unclear what the s refers to), often this is 2x Q n The minimum flow rate is lowest possible flow while still meeting the accuracy rating (see later) is variously referred to as Q min and q i The turndown ratio or dynamic range or measuring range is the ratio of (Q n/q min) or (q p/q i) Example Another term sometimes used is the starting flow, meaning a flow rate that will cause the meter to register a flow condition and is (unsurprisingly) referred to as Q s. Between Q s and Q min, the accuracy may be poor but this is likely to be of only minor concern as the thermal power at such a low flow rate will also be small and is expected to be only a minor contributor to overall thermal energy. A solar thermal system has a volume flow rate of 0.4 L/s (1.44 m 3 /h) and the pipe size is 32mm. The selected thermal energy meter selected for flow has a Q p rating of 1.5 m 3 /h. By comparison, if meter had been selected to match the line size it would have a Q p of 3.5 m 3 /h. While this is nominally a fixed flow system, if flow was variable, the latter selection may have accuracy problems at very low flows. Several flow measurement types used as part of TEM package. The most common are: Standard water meter, typically single-jet or multi-jet bit could be turbine type if large or even positive displacement, generally lower cost (due to production volume), relatively simple and robust, higher flow resistance, lower accuracy and lower turndown ratio Ultrasonic, generally higher in cost but simple, robust, lower flow resistance, higher accuracy and higher turndown ratio Less common types include electromagnetic and thermal, with the latter measuring mass flow directly. Accuracy is usually referenced to European standard EN 1434 which has three accuracy classes, unsurprisingly called Class 1, 2 and 3 which notionally mean accuracy at the nominal flow rate of ±1%, ±2% and ±3% respectively with an increase in the accuracy band at lower than nominal flow. At one-tenth of Q n (0.1Q n) or Q t (10:1), the Class 1, 2 and 3 accuracy widens out to ±1.1%, ±2.2% and ±3.5% respectively and at one-hundredth of Q n (0.01Qn), the Class 1, 2 and 3 accuracy widens out to ±2%, ±4% and ±8% respectively. This is shown in the following graph (caution, note logarithmic horizontal axis).
Reference 5 Thermal Energy Metering It is apparent that accuracy is slightly better at the upper end of the flow range, so selecting an oversized meter which leads to measurements at lower flow rates will degrade accuracy. A bit more Many meters are rated with turndown ratios of 100:1 or even higher but this may be misleading as accuracy may be quite high at very low flow conditions. This means the ratio of nominal flow to minimum flow for a meter operating at 2% and 1% of nominal flow respectively: For Class 1: the allowable accuracy band at these flows are ±1.5% and ±2.0% For Class 2: the allowable accuracy band at these flows are ±3.0% and ±4.0% For Class 3: the allowable accuracy band at these flows are ±5.5% and ±8.0% These are the upper bounds for the classification at this flow ratio, so in actual practice the device will achieve a better accuracy. Not all manufacturers quote an accuracy class. Sometimes the actual ± % range is quoted for the flow range in question. Also, the actual accuracy achieved may vary depending on fluid temperature. The maximum and minimum acceptable temperatures are usually quoted but it is unclear whether this means the accuracy class claimed is achieved across the full temperature range. Related factors are that the density of water varies so volume measurements converted to mass will be different at low and high temperatures, being larger at higher temperatures and smaller at lower temperatures. And, now for some more 1000 L measured at 20 o C has a mass of 998.2 kg. 1000 L measured at 10 o c has a mass of 999.9 kg or 0.2% more 1000 L measured at 90 o C has a mass of 965.25 kg or 3.3% less Refer Calculator section for further consideration of this aspect.
Reference 6 Thermal Energy Metering Class 2 and 3 devices seem to be the most common for general use. Class 1 is expected where higher accuracy is essential. In some cases, the body of the flow measurement unit will include a temperature sensor pocket which simplifies overall system installation. Temperature Measurement The most commonly used temperature sensors are passive resistance types such as platinum RTD (resistance temperature detector 2 ) either Pt100 or Pt500 which refer to the resistance in ohms () at 0 o C. Because the resistance of the lead has an effect, it is built into the calibration of the RTD. This means that extending factory-calibrated RTDs is possible, it will lead to an error in measurement. Even so, it may be possible to extend the length of both RTDs in a pair by the same amount and achieve an accurate assessment of t but this may void manufacturer s warranties. An alternatives to the simple 2-wire connection type noted above is a 4-wire RTD where the second pair provides automatic compensation for changes in cable length and cable type. Thermocouples may also be used but are uncommon in this application. Rather than specifying the accuracy for individual temperature measurement, because the aim is accurately measure temperature difference, it is common for matched pairs of temperature sensors to be calibrated together. For such matched pairs, the following may be applied: Class B: linear from ±0.30K at t of 0K to ±0.8K at t of 100K Class A: linear from ±0.15K at t of 0K to ±0.35K at t of 100K Sensors may be direct insertion type which means they are in contact with the fluid and so measure directly (faster response), or indirect insertion type which means they are within pockets extending 2 Similar but slight different devices include thermistors and integrated circuit type. Some are referred to as NTC or negative temperature coefficient meaning their resistance falls with rising temperature unlike most common RTDs (such as Pt100 and Pt500) whose resistance rises with rising temperature.
Reference 7 Thermal Energy Metering into the fluid and not in contact with the fluid itself and so measure indirectly (slower response). In the latter case, good thermal contact between the sensor and inside of the pocket wall is essential. A heat conducting material may be used for this purpose. In industrial applications, additional common temperature sensor types are active or powered type such as current loop (4-20 ma) or Calculator At its most basic this has inputs to accept signals from the flow measurement and the two temperature sensors. Levels of complexity may be available including: Simple communication via pulse data outputs, typically for energy and volume Simple pulse inputs, typically to collect data from nearby pulse output meters such as water, gas and the like, basically a counter with off-meter scaling applied Daily totals for at least a month, monthly totals for at least 12 months which may be useful for cost allocation Complex communication via a hard-wired communications port, with various topologies (such as RS485 multi-drop) and protocols (such as Modbus, M-Bus) Complex communication via a un-wired communications port, with various topologies (such as point-to-point, mesh) and protocols (such as wireless M-Bus, ZigBee) Complex communication via a hard-wired communications port through a protocol conversion process to allow communication over LON, BACnet, KNX and other networks both locally and remotely across the internet or intranet Walk-by, drive-by data collection An important aspect to consider is power supply. If standalone (not on a wired bus), the options available are battery power, mains power or mains with battery backup. Battery power alone is not recommended except under special circumstances. If on a wired bus, the options available are buspowered, bus-powered with battery backup for on-board data, or mains powered. What is available will vary from supplier to supplier. While it is anticipated that some correction is built-in to the calculator for variations in fluid density and specific heat with temperature, this becomes potentially problematic at higher values of t. For example, using 20 o C as a reference, c p 4.183 kj.kg.k and of 998.2 kg/m 3, so c p x is 4175.47: At 10 o C, c p 4.193 kj.kg.k and of 999.9 kg/m 3, so c p x is 4192.58 or 0.4% less than reference At 90 o C, c p 4.208 kj.kg.k and of 965.25 kg/m 3, so c p x is 4061.77 or 3.1% more than reference The details of how correction is applied is not readily apparent from equipment suppliers literature.
Reference 8 Thermal Energy Metering Integration Unless the user has special knowledge and capability, the most reliable approach is to source from a single supplier, all the components that are known to operate together to create an integrated package. This simplifies the provision of support, troubleshooting, programming and warranty issues. It is also important that if the device is to be connected to a third party system such as a building automation system or similar, the third party provider has the necessary skills, knowledge and experience to create an integrated system. About the Author Robert Alexander is the Senior Energy Efficiency Engineer, at Genesis Now, and has specialised in energy efficiency since 1983. Genesis Now, established 1991, is an energy efficiency consulting engineering and project implementation company, based in Melbourne, Australia. Enquiries: Genesis Now www. http://genesisnow.com.au/contact/ phone 1800 22 99 11 (within Australia)