Power consumption is used for the derivation of indirect emissions and annual running cost. The table summarises the conditions used to determine the compressor COP (Coefficient of Performance) and hence power input. It also shows the additional power input for fans, lighting and ancillary pumps. These are included in the Annual Running Cost and TEWI assessments. If the cooling loads are changed all power values are factored accordingly.
The LT evaporating temperature is -35°C except for R744 cascade and secondary systems where an allowance is made for better heat transfer capability giving -32°C for similarly sized coils. For single stage LT, Condensing temperature is 25°C for Northern Europe and 30°C for Southern Europe. With Secondary systems, glycol circulates at -3°C and CO2 at -1°C. The zero condensing temperature in both cases reflects the better heat transfer with CO2.
The MT evaporating temperature for DX systems is -6°C. This is based on an air temperature of +5°C, with a deltaT of 11K to allow for evaporator superheat. For cascade, the LT R744 is condensing at +2 °C, hence the MT evaporating temperature is -9°C. For the secondary system, glycol is at -3°C for an 8K evaporator deltaT (superheat allowance not necessary), resulting in an evaporating temperature of -9°C. For pumped CO2 at -1°C the evaporating temperature is -7°C. The condensing temperature is 25°C for Northern Europe and 30°C for Southern Europe
Compressor data is taken from Copeland Selection Software 7 and the software/catalogues of other compressor manufacturers. We have chosen typical semi-hermetic reciprocating and scroll compressors using the latest available technology. COP is defined as the compressor COP, i.e. the ratio of compressor published capacity and power at the evaporating and condensing conditions specified in the tables above.
R744 transcritical technology is at an early stage of development and, whilst compressor data is available, the method of application and control can have a significant influence on system efficiency. Operation in the subcritical mode can offer better efficiencies than today's conventional systems. This better efficiency is needed to offset the low transcritical efficiency that comes with high ambient conditions. Location therefore plays an important role. The warmer the climate, the longer the system will operate in transcritical mode. In this model, we have taken a “best case” scenario for the R744 transcritical system.
Studies have shown that optimised R744 transcritical systems serving both the LT and MT loads can operate at average COPs similar to semi-hermetic reciprocating R404A systems in the Northern European climate. This is the comparison normally made when assessing the merits of a totally natural refrigerant solution. For this to happen, advantage is taken of lower R744 pressure drops and better heat transfer properties, both of which have the effect of reducing the compressor “lift”. Subcooling is also beneficial with R744 and would normally be required for the optimised R744 system to reach the COP parity mentioned here.
For the R404A system we have assumed that the head pressure is set at minimum 20°C condensing at all ambient conditions below 10°C whereas the R744 head pressure is allowed to float down to a much lower temperature. The graph illustrates the relative change in COP with outdoor temperature. The Base Case with scroll technology shows an average MT COP improvement of approximately 10% when compared to typical reciprocating compressors. We have assumed that the R744 solution can reach the same average COP as a typical reciprocating solution in the Northern European climate. For Southern Europe, the R744 transcritical system is assumed to operate at 10% lower average COP than the HFC MT system, and this is to account for more operating time at transcritical conditions.