ASHRAE Journal - February 2009 - 37

40 operation of the plant, called “auxiliaries,” 104 such as evaporator fans, antimisting resis95 35 tors, cabinet lights, etc. These are quite 86 30 constant with EEVs and TEVs. They can 77 25 be neglected in PES calculation. 20 68 Electrical consumption for the com°C 15 59 °F pressors has been calculated by multiplying the values of Figures 2 and 3 by the 50 10 total cooling capacity of the two circuits 41 5 (respectively 75 and 324 kW) and by Milan 0 32 the number of hours for each outside Rome –5 23 Trapani air temperature value. They are in good 8 The 14 –10 agreement with experimental data. 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 values increased from Milano to Trapani, Hours for low- and medium-temperature circuits, Figure 4: Outside air-temperature distribution for the three considered climates (regression but more for the latter in relative terms. In of hourly data). fact, as written before, the sensitivity to the climate is higher for medium temperature connected to the excluding the costs of monitoring. Total cost is composed of stronger influence of a different condensation temperature in the components (mainly EEVs) and labor. The cost of the EEV smaller temperature difference of medium temperature. solution is approximately $445 for each of the 118 display Heat recovery (obtained by means of regression functions cabinets in the studied supermarket. This results in a $52,450 on the basis of experimental data8) increases from colder to total investment cost for the plant. Total cost is lower when milder climates (due to a larger number of hours with higher EEVs are directly installed by cabinet manufacturers. outside air temperatures, therefore, with higher refrigerant Electrical energy cost is fixed at $0.127/kWh, interest rate at condensation temperatures). 5% and an investment period of 15 years. Table 3 reports the In terms of primary energy saving (the last three columns in results of the economic analysis for the “base case” described Table 2), consider that TEV allows a higher heat recovery (and above, in terms of annual savings, net present worth (NPW), and therefore the item is negative), because of the larger quantity of heat discounted payback period (DPP). Because the NPW is always recoverable with TEV due to the higher condensation pressure with positive, investment is advantageous for all the climates. Initial outside air temperature lower than 37°C to 38°C (99°F to 100°F) costs are recovered in a short time period, about 1.4 years for (by far the most frequent condition for all the three climates). It is Milano and a little more for the other climates. worth remembering that the higher condensing pressure with the A sensitivity analysis can illustrate the possible results for TEV is due to the constraint of a higher pressure differential across parameter values diverging from the previous hypothesis. An the TEV that provides proper refrigerant flow control. increase of 50% in the investment cost slightly influences the Considering all the energy elements (compressors, condenser profit of the intervention just as a doubling of the interest rate. fans, auxiliaries, and heat recovery), Figure 5 reports the primary The most influential parameter is the electricity tariff, where energy saving for the three climates. Note that, for this application, halving that would roughly double the payback period (doubling total PES for compressors consumption is around 36% for Milano, the tariff would halve the payback period). This would not alter and decreasing to 29% for the milder city of Trapani. Considering the overall advantage even if you do not consider a probable all the electrical consumption (power and auxiliaries), energy sav- increase of the tariff with time. ing is respectively 22% and 18.5%. It does not suffer too much of the greater heat recovery with TEVs for this particular application Conclusions (PES decreases only by 1% taking into account heat recovery). Using EEV technology with display cabinets of a large supermarket demonstrated considerable energy savings due to the suAn Economic Evaluation perior control characteristics of the EEV and the favorable type The previous analysis demonstrated an energy advantage of application, (operating conditions in which the condensing of the system equipped with EEV in this application for hot temperature is allowed to drop with the outdoor temperature). climates, an economic analysis is necessary to assess the real A simulation model demonstrated that similar advantages are profit of the replacement of the valves in an existing plant. The possible under different climate conditions. For this particular parameters to be fixed are the investment costs, the unitary cost case, with heat recovery on desuperheating of refrigerant, only of electricity, and the discount rate for the investment. Another a slight penalization is introduced. As the required investparameter is the useful life of the investment. The payback pe- ment cost is not particularly high, energy savings also allow riod is far shorter than the expected life of the equipment. important economic savings and a short payback period (1.5 Investment costs of the innovative system were obtained on years and probably shorter if EEVs are installed directly by the basis of the costs met in the retrofitting of the supermarket cabinet manufacturers). It is surprising that such a small piece February 2009 ASHRAE Journal 37

ASHRAE Journal - February 2009

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