| Front page | | Contents | | Previous | | Next |

Appendix D

Reference shop. Load determined by scaling of appliance length/store size

Introduction

In order to compare the energy consumption of the different supermarkets, it was necessary to refer to a standard shop. The circumferences of the freezing and cooling gondolas, the lengths of the cooling racks and counters, and the volumes of the freezing and cold stores were set for the standard shop. In order to get the variation in the load included in the energy consumption, the ratio of the actual load to the nominal load was assumed to be constant. The nominal load was determined on the basis of the information from the manufacturer concerning the refrigeration requirements for the appliances and the stores.

Procedure for calculating comparable energy consumptions

The energy consumption was calculated on basis of the hourly means of measured evaporation and condensation pressures, compressor suction temperatures, and used compressor capacity, and the data from the compressor manufacturers. The power was corrected to power at a condensation temperature of 30°C to eliminate the influence from different operation strategies on the condenser side (e.g., heat re-circulation).

Data on the nominal refrigeration requirements of the individual refrigeration appliances and cooling/freezing storages is given for each supermarket. These data are converted to a nominal consumption value per appliance meter and per m ³ of store.

Since the nominal consumption per appliance meter and per m ³ of store was different for each supermarket (the nominal consumptions of the appliances in the newest supermarkets are lowest), a mean for the four supermarkets was calculated. A reference supermarket was defined with freezing and cooling gondolas with a 50m circumference, 35m cooling racks, 12m counters, 350m ³ cooling storage and 100m ³ freezing storage. This reference supermarket corresponds to a mean of the four supermarkets.

The energy consumption of each supermarket was converted to a consumption of the reference supermarket. This was done on the assumption that the relation between the nominal consumption and the actual consumption is the same for the actual supermarket and the reference supermarket. This means that the evaporation temperature of the reference supermarket is the same as the measured evaporation temperature.

The reference loads on the cooling and frost sections of the cascade systems were determined first. The load on the high-temperature section is the sum of the load from freezing plus the power consumption of the low-temperature compressors and the load from the cooling appliances.

Calculation of energy consumption based on data from the compressor manufacturers

"Measured" actual power consumption

Based on the measured data the refrigeration capacity and the power consumption at the reference condensation temperature can now be calculated as follows:

Actual volume flow is calculated on the basis of measured pressure ratio and suction temperature using the expression:

with:

: Actual volume flow through the compressor

: Volumetric efficiency

P_c Condensation pressure

P₀: Evaporation pressure

T_c: Condensation temperature

: Geometric stroke volume of the compressor

N_com: Number of compressors

I_ComCap: compressor capacity in use

The mass flow is, therefore:

with:

v(T₁,P₀): Specific volume at suction stop valve

T₁ Temperature at suction stop valve

P₀: Evaporation pressure

The "measured" refrigeration output is thus:

with:

h₁(T₁,v₁(T₁,P₀)): Enthalpy at suction stop valve

h₃(T₃): Enthalpy after condenser

T₃: Temperature after condenser

And the corrected "measured" power consumption is:

with:

h_2cor,is(T_2cor,is,v_2cor,is) Enthalpy after compressor at isentropic compression

: Isentropic efficiency

P_c.cor: Condensation pressure at reference condensation temperature

T_c,cor: Reference condensation temperature

Reference load

Conventional system

The refrigeration capacity for the reference supermarket with separate circuits for cooling and freezing are determined using the following procedure:

with:

and the power consumption is determined as:

with:

Cascade system

The refrigeration capacity and power consumption of the freezing section of the cascade system is calculated as indicated above when the system has separate circuits. The power consumption of the freezing section of the reference system is thus:

with:

The load from the cooling appliances and storage of the cascade system in the reference system is determined as:

with:

The total power consumption by the high-temperature section is determined as:

The total power consumption of the cascade system is, therefore:

Results

Figure D1 shows a comparison of the energy consumption per day for September 2003. The situation was quite similar during the rest of the measuring period.

Figure D2 shows a comparison of the power consumption (hourly means) for a 24-hour period (1 September 2003)

Figures D1 and D2 show that the energy consumptions of the reference shop with data from ISO-1 and ISO-3 are about 2/3 of the energy consumptions of the reference shop with data from ISO-2 and ISO-4. A critical examination of the calculated energy consumptions of the reference shop, however, shows that the loads are not the same for the reference shop with data from the four supermarkets, as shown in figures D3 (load on cooling) and D4 (load on freezing). ISO-1 and ISO-3 have loads on cooling which is about 2/3 of the loads for ISO-2 and ISO-4. The load on freezing for ISO-3 is considerably below the loads for ISO-1, ISO-2, and ISO-4. This difference in load is partly explained by difference in relative load, as shown in figures D5 and D6. These figures show that especially ISO-4 has a load profile which differs considerably from those of the other three. These figures show that ISO-4 especially has a load profile, which is very different from those of the other three. This difference must be caused by a scaling factor, the nominal load, which is too uncertain.

Conclusion

It must be concluded that a "simple" scaling based on nominal refrigeration requirement does not produce a true picture of the energy consumptions of the supermarkets because of uncertainty regarding the nominal refrigeration requirements.

Figure D1. Energy consumption per day in reference shop based on measured energy consumption (month)

Figure D2. Power consumption in reference shop based on measured energy consumption (day)

Figure D3. Cooling load in reference shop based on measured load (day)

Figure D4. Freezing load in reference shop based on measured load (day).

Figure D5. Relative load cooling (day)

Figure D6. Relative load freezing (day)

| Front page | | Contents | | Previous | | Next | | Top |

Version 1.0 September 2004, © Danish Environmental Protection Agency