District Energy Efficiency by Flow Metering
Energy efficiency is becoming increasingly important as energy costs increase and carbon footprints considered, this is true across all organisations in both public and private sectors and affects all aspects of our lives especially in in the built environment. 40% for energy consumption and therefor carbon emissions in Europe and the United States emanates from how our buildings are lit, cooled and heated. Small changes on a big scale can have a major impact on energy performance.
This case study looks at energy efficiency and how energy monitored by flow metering can help to realize energy saving targets and submetering to enable cost centred billing within building complexes providing accurate information on consumption.
Introduction
Building Management Systems are designed to monitor and improve energy efficiency in buildings, control the energy supplied to the building and are widely used adjust lighting intelligently switching on only when required ensure heating, cooling and air conditioning operates efficiently.
Energy Management
Energy efficiency is simply management. Regardless of the cost to build and modify a building and the use of energy-efficient equipment, without proper management you will not get the full benefit.
I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind.
Lord Kelvin
If we can measure we can improve and control which is true for a single dwelling, a large building complex, shopping centre, a university campus, office block, airport or a military base. The difficulty of solving problems increases as additions are made and the size of the project grows. Multiple ownership or different cost centres exacerbate these challenges often requiring multiple measurement points.
A single building requires a simple heating, cooling and air conditioning system with heat coming from a boiler generating hot water for a circulating radiator system and air conditioning provided by a separate refrigeration system on a separate circuit. At the other end of the scale and large complex would have its heating and cooling requirements met by a single or district energy system. District energy systems are networks of hot and cold water pipes, typically buried underground, that are used to efficiently heat and cool buildings. They use less energy than if the individual buildings were to each have their own boilers and chillers but they also offer challenges when it comes to measuring and monitoring.
Many large building complexes meet their heating requirements with Combined Heat and Power (CHP) plants allowing the recovery of waste heat to be captured. Examples exist where >50% of the initial heat generated is wasted but by use of heat recovery systems efficiency can be increased to as much as 90% providing large financial and environmental benefits.
Regardless of the energy source; gas, oil, electricity or renewable there are many parameters that require measurement and monitoring to allow efficient heating and cooling performance and provide accurate local billing. Among the most important parameters are flow and temperature as they are used to calculate the energy consumption.
Flow Metering
Selection of the right Flow meter can be a difficult task with each technology having pros and cons and although accuracy is a prime consideration there are other factors that will affect the final choice.
- Do they cause a pressure drop – this will require increased pump pressure and increased cost?
- Is the meter easy to install and inexpensive to maintain?
- Can it handle a wide range of flows?
- Can it be integrated with the building complex energy management system?
- Does the meter have moving parts that can wear and if not maintained become less accurate and false readings
- Can it provide true energy consumption details?
- Does it need calibration?
And finally, what are its capital and operating costs?
Types of Flow meters used for energy flow measurement
Turbine Flow Meters
These are simple mechanical devices that measure flow simply by recording the action of the flow on a movable component. The most common are turbine meters where the linear velocity of the fluid or steam rotates a primary impeller element. In the same way as orifice plates, turbine meters are placed in the flow and this obstruction creates a pressure drop that introduces an energy loss. The rotors on a turbine meter can require maintenance and replacement after time.
Installation on existing pipes can be difficult and time consuming, involving shutting down the process, draining the line, cutting the pipe and welding on flanges.
Orifice Plate Flow Meters
Orifice plate flow meters use the same principle as a Venturi nozzle. The differential pressure across the orifice is proportional to the square of the fluid velocity. This fluid flow figure can be given in volumetric or mass flow rate depending on the calculation applied. Orifice plates are simple, rugged, reliable and inexpensive and have no moving parts. However, they are not as accurate as other technologies and cause a pressure drop requiring increased pump pressure that in turn contributes to overall energy loss. The orifice plate can, in time, erode causing inaccuracy. These flow meters also cannot handle a wide range of flows (known as a low turndown) with a maximum ratio of just 10:1 between higher and lower flow rates.
Installation on existing pipes can be difficult and time consuming, involving shutting down the process, draining the line, cutting the pipe and welding on flanges.
Magnetic Flow Meters
Magnetic flow meters are commonly used for measuring water but cannot measure compressible fluids or non-conductive liquids. They use Faraday’s second law of electromagnetic induction creating a magnetic field in the conductive liquid. The flow of the liquid through this field is sensed by electrodes located on the pipe walls which produce a voltage signal. The voltage generated is proportional to the movement of the flowing liquid, according to the equation:
Voltage = B x v x d
where
B is the magnetic flux density,
v is the velocity of the fluid
d is the diameter of the pipe.
Magnetic flow meters provide a typical accuracy of +/-0.3% of reading and do not create a pressure drop but can be subject to erosion and scale build?up. Installation on existing pipes can be difficult and time consuming, involving shutting down the process, draining the line, cutting the pipe and welding on flanges.
Vortex Flow Meters
Vortex meters measure flow rate by measuring the frequency of vortices shed off a bluff body located in the flow stream. The frequency of vortex shedding is measured by a piezo electric crystal which produces a small voltage pulse every time a vortex is created. The frequency of these pulses is proportional to the fluid velocity.
Vortex meters offer a cost-effective, accurate and reliable measurement for volumetric and mass flow of both water and steam. They do not need
re-calibration and can offer a volumetric flow rate repeatability of +/-0.1% of reading and a mass flow rate repeatability of +/-0.2% of reading. The bluff body used to create the vortices naturally creates a slight pressure drop that may increase energy usage, but installation is relatively simple.
A major advantage of vortex meters, such as the multivariable MV80 and MV82 from Panametrics are that they incorporate an integrated resistance temperature detector(RTD) to measure temperature. This can then be combined with the flow rate and the pipe area to for calculating energy rates in BTU/hr, KJ/hr, calories/hr or other selectable energy units. The MV80 and MV82 meters also feature an integral pressure transducer to offer better energy usage calculations for steam. Integration with simple or sophisticated energy management systems is possible by means of HART® or Modbus communication protocols with addition to outputs for totalized mass and alarm settings and field-configurable electronics deliver up to three analogue 4-20 mA outputs of five process measurements. A turndown of more than 100:1, dependent on application makes the these vortex meters suitable for a wide range of flows especially important in a building complex, especially for steam, where flow rates vary considerably between seasons.
Installation on existing pipes can be difficult and time consuming, involving shutting down the process, draining the line, cutting the pipe and welding on flanges.
Ultrasonic Flow Meters
Ultrasonic flow metering is thought of as a relatively new technology however ultrasonic flow meters have been used for liquid flow measurement for more than 40 years. Ultrasonic flow meters using the transit time flow measurement technique are the most common.
Ultrasonic transducers are installed in or on the pipe, one upstream of the other. By measuring the time of flight difference between the ultrasonic pulse sent from one transducer in the flow direction and an ultrasonic pulse sent from the other transducer opposite to the direction of flow, flow can be calculated. Both transducers operate in transmit and receive mode. Calculating the difference between these two time intervals is a measure of the average velocity of the fluid along the pipe. By multiplying the calculated velocity by the cross-sectional area of the pipe the volumetric flow can be obtained. Mass flow is calculated by multiplying this figure by the fluid density.
There are in-line and clamp-on versions Ultrasonic Flow Meters. In-line flow meters have the transducers installed in a flow cell or spool piece inserted into the pipe or located directly inside the pipe. Clamp-on meters, as the name suggests, are quick and easy to install and simply clamp onto the outside of the pipe. They do not involve modification of existing pipe shutting down the process, draining the line, cutting the pipe or welding on flanges.
Ultrasonic Flow Meters for Water and Steam
Transit time ultrasonic flow meters are widely used on both water and steam. As well as calculating actual flows they also feature the signal processing power to provide calculation of energy delivered or removed from a particular location.
Thermodynamically, this calculation relies on the equation:
Delta Q = Vap(hs-hr)
where
Delta Q is the rate of heat exchange
V is the fluid velocity
p is the fluid density
hs is the enthalpy of the fluid supply
hr is the enthalpy of the fluid exit.
Enthalpy is the total heat of the fluid and is dependent on the temperature and pressure. It is a measurement of amount of energy in kilojoules or BTU, contained in 1 kg or 1 lb of the fluid. For flows in pipes pressure is constant so any change in enthalpy is purely dependent on the change in temperature between inlet and outlet.
Ultrasonic flow meters, such as those from Panametrics contain built-in density information and enthalpy tables for water and steam allowing exact calculation of energy usage in a range of engineering units. They also feature RTDs to provide accurate measurement of relevant temperatures and many offer multiple outputs to allowing integration with building facilities management systems using common protocols such as HART® or Modbus protocols.
Conclusions
Energy efficiency is critical and accurate monitoring is key to any energy efficiency strategy both to identify efficient energy usage and to ensure that discrete zones are accurately billed for their energy consumption. In building complexes such as university campuses and government facilities, energy monitoring relies heavily on the measurement of the flow of the heating or cooling media.
There are a wide range of flow metering technologies for different applications, the selection of which requires research and comparison however in the simplest of terms the instrument needs to be accurate and reliable. In an ideal world flow metering would not induce a pressure drop, (ironically a source of energy loss) and it must be easy to use and maintain. In some applications, especially in older buildings, ease of installation is important. Turndown (rangeability) of measurement is also important especially where steam monitoring is being undertaken and where there is a wide difference in the flow rate i.e. summer v winter. Finally integration with modern building facilities management systems can often be required.
Panametrics and RS Hydro understand the need to accurately monitor, measure and analyse flow efficiency in order to reduce energy costs and reduce environmental impact. We have been helping our customers for over 20 years providing unbiased advice on how to select the right meter to get critical information to improve their heating and cooling systems and to track energy usage. As a result, they continue to benefit from improved billing accuracy reduced maintenance costs and decreased cost of ownership.
