Cold Link Africa January / February 2020 | Page 37

CONTRIBUTORS INCORPORATING COLD CHAIN When it comes to COP, I say “don’t tell me, show me!”, because hundreds of scientific in-field test data using ClimaCheck shows that a tiny fraction of systems operate at the exact conditions used in the factory. The data also shows how insufficient maintenance, sub-standard installations, poor control set-up, lack of or incorrect commissioning procedures, amongst others destroys efficiency as well as reliability. So, next time someone tells you the COP of their system is six (or any other number), ask them: 1. “When was it six?”; and 2. “How did you measure that?”. ‘HIGH-EFFICIENCY’? COMPARED TO WHAT? It is extremely common to hear the phrase ‘high-efficiency’ in discussions in the HVACR industry. You will be hard- pressed to observe any discussion that does not contain this phrase. In today’s market, is it even possible to buy a ‘low- efficiency’ or for that matter, ‘standard- efficiency’ system? If this is the case, why do we use this phrase so frequently? By definition, high-efficiency is a relative term, so when a manufacturer or supplier quotes ‘high-efficiency’ what are they comparing it to? A cynical person might say that they are referring to the opposition’s equipment or, very often, the existing 15-year- old system, which is in perfectly good working condition, but which they intended to replace. In this example, ‘high-efficiency’ is relative to a very low base. Does a 10% increase in efficiency compared to this old system qualify as high efficiency? Perhaps 20%? This doesn’t make sense, so let’s stop over- using the phrase ‘high-efficiency’. TOP 5 EFFICIENCY KILLERS The vast majority of systems have a savings potential of 10% to 30%. Of those opportunities, 80% are operational in nature, in other words, no capital investment is required and, therefore, this is a desirable return on investment. Notwithstanding this, there are many challenges and barriers to the market. These are some of the most common: Business as usual. There is little or no incentive for most role players in the HVACR industry to directly change the current business model because efficiency is seldom measured and therefore, cannot be used as a performance benchmark. “If it ‘ain't broke, don’t fix it”. Run-to-fail maintenance strategies are common- place and result in avoidable failures and lost efficiency. We see many examples where a loss of efficiency due to (for example) small maintenance issues have lost efficiency for several years. “I have a Scada/BMS/Dashboard system; therefore, I know the efficiency of my system at all times”. Most of these systems generate data about temperature, pressure and setpoint alarms – they do not calculate the efficiency. “My system is effective; therefore, it is efficient” — a common misconception, especially amongst end-users. If your COLD LINK AFRICA • January/February 2020 office is at the required air temperature (effective), it does not necessarily mean it is efficient. “The system was commissioned when it was installed; therefore, it is efficient.” The majority of systems are either not adequately commissioned or not commissioned at all (the phase ‘retro-commissioning’ has now become a commonly-used term). Proper commissioning is one of the most important contributors to the efficient and reliable operation of any cooling system. MEASURING YOUR PLANT The system identifies inefficiencies, documents baseline performance and provides early warning for system weaknesses. You can compare the cost of energy optimisation against calculated savings once a performance baseline is documented, ensuring your investment is paying back. Studies and practical experience show that the system can reduce energy consumption by 10-30%. As 20% of world energy consumption is used by refrigeration processes, these energy reductions directly affect global carbon dioxide emissions. HOW IT WORKS The ClimaCheck measuring tool uses a process called the ‘internal method’. This means that the performance and efficiency of any vapour compression cycle can be calculated on-site and in real-time with non-invasive temperature and pressure probes. Think of it as taking a laboratory to the chiller or refrigeration system. The internal method is based on the possibility to thermodynamically determine the refrigeration process by assessing the specific enthalpy changes in various parts of the refrigerant system. (Berglöf, 2004). QUANTITIES THAT MAY NEED TO BE MEASURED ARE: • • • Surface temperatures in the refrigerant system Refrigerant high (condensing) and low (evaporating) pressure Electric powers (compressors, pumps, fans, etc. according to system boundary for calculation of COP. Knowing the temperature, pressure and type of refrigerant at a particular location, it is possible to get the value of the specific enthalpy in a table or chart or to calculate the value by means of known correlation or by means of programmes. In a simple, one-stage refrigeration cycle measurement of the condensing and evaporating pressures, the outlet and suction temperatures of the compressor and the sub-cooled liquid temperature after the condenser will suffice. These measurements, i.e. two pressures and three temperatures provide sufficient information to visualise the process by means of points 1, 2, and 7 on the diagram. Assuming isenthalpic expansion point 8 will also be known. Figure 13 The refrigeration process in a diagram of specific enthalpy versus pressure. By means of computerised equipment, it is possible to use the measured pressures and temperatures to calculate the specific enthalpies on the saturation curve as well as in the regions of superheated vapour and sub-cooled liquid. When the process has been mapped it is possible to calculate the heating or cooling coefficient of performance, e.g. • For cooling the coefficient of performance COPc in a corresponding way is given by: • The method can either be used to directly determine the coefficient of performance, using a known value of the loss factor, or to study performance changes after calibration by means of a parallel external measurement or simply to assess relative changes. CLA Equation 28 Where h 2 - h 7 represents the heat delivered by the refrigerant to the condenser and h 2 – h 1 represents the work supplied to the refrigerant by the compressor. Unfortunately, the reality is a little more complex as not all of the compressor work will result in a specific enthalpy increase from point 1 to point 2 owing to thermal losses to the ambience. In a simple, hermetic compressor, the losses P. loss may be expressed as a fraction f of the motor power input P e,m . This enables a calculation of the refrigerant mass flow rate q m,R according to: • Equation 32 As 20% of world energy consumption is used by refrigeration processes, these energy reductions directly affect global carbon dioxide emissions. Equation 29 Then the heat pump heating coefficient of performance COP h is given by • Equation 30 i.e. REFERENCE : • • Equation 31 Anna-Lena Lane, Jessica Benson, Lina Eriksson, Per Fahlén, Roger Nordman, Caroline Haglund Stignor, Klas Berglöf, Guy Hundy. (2014). Method and guidelines to establish System Efficiency Index during field measurements on air conditioning and heat pump systems. SP Technical Research Institute of Sweden; Energiteknik, p. 13, 31, 32. www.coldlinkafrica.co.za 37