1. “Improving The Energy Efficiency and Milk Quality with CO2 Refrigeration Systems with Heat Recovery on New Zealand Dairy Farms”
Klaas Visser
Principal
KAV CONSULTING Pty. Ltd. Dip.Mar.Eng. (NL)
Hon.M.IIR, F. Inst R, M.IIAR, M. ARA, M.KNVvK, Meurammon.
PO. Box 1146, KANGAROO FLAT, VIC, 3555 AUSTRALIA
Tel:

2. Introduction
The following new regulations promulgated by the NZ Ministry of Primary Industries require the following time temperature cooling performance to be achieved effective 1/1/2018.
The storage vat milk temperature must not exceed 10°C within 4 hours of the commencement of milking.
The storage vat milk temperature must not exceed 6°C within 2 hours of completion of milking.
The milk must be kept at 6°C maximum until it is collected or by the next milking.
Milk added to the vat at the next milking must not raise the milk temperature in the vat to above 10°C.
The next potential problem is the G20 mandated 79% HFC phasedown below mid 2013 levels by 2030 under the auspices of the Montreal Protocol (MP). The HCFC phase out continues.

3. Electrical energy consumption on a typical New Zealand dairy farm
Energy Consumption on a Typical Dairy Farm
Water
Pumping
Water Heating
10%
Milk Chilling
32%
21%
11%
26%
Milking
Other
System
(Ref.

4. Performance of a commercially available semi-hermetic R134a compressor with 5.5 kW motor and a swept volume of 33.1 m3/h. Liquid subcooling takes place in a SHEX.
COPS of varying Saturated Suction Temperatures (SST), C
(1)
-10 -5 -1 +5
Saturated
Condensing
Temperature
(SCT ), C
LSC K
(2)
LSC K
(2)
LSC K
(2)
LSC K
(2)
COP
COP
COP
COP 20
4.06 5
4.91 4
5.76 3
7.47 3 25
3.68 6
4.30 5
4.97 4
6.28 3 30
3.17 7
3.79 6
4.35 5
5.4 4 35
2.82 7
3.31 6
3.82 6
4.7 5 40
2.51 8
2.95 7
3.37 7
4.08 6 45
2.25 9
2.62 8
2.94 7
3.6 7
(1) Suction vapour inlet condition +20°C in all cases
(2) Liquid SubCooling, K – achieved in SHEX0

5. Refrigeration Condensing Methods and Advantages/Disadvantages in Melbourne at 35°C Dry Bulb, 22°C Wet Bulb
Parameter
Refrigerant Condensing Method
Evaporative
Condensers
No.
Description
Air Cooling
Cooling Towers
Commercial and
Large Scale AC & Low 1
Refrigeration Application
Industrial
Small Scale AC
Charge
Industrial
Temperature + 3 to
Ambient Air Wet Bulb
10K
Air Entry to Leaving
Water Approach
Coolant Temperature Worldwide
Ambient Air
Dry Bulb
Ambient Air Wet Bulb
Temperature 2
Maximum
Temperature CO
Condensing Temperature/Gas 2 3
Cooler Exit in Melbourne Ambient
+40 - 45 C
+30 - 37 C
+28 - 30 C
+35
CDB, + 22 C 4 CO
Critical Temperature, C
+31 C
+31 C
+31 C 2
Mostly Transcritical,
but Subcritical with a
Low Air WB Entry
Approach
Subcritical
All the Time 5 CO
Type of Operation
Transcritical 2
Moderate Energy
Consumption
Low Energy
Consumption 6
Advantages
Low Costs
High Energy
Consumes Water and
Chemicals, Legionella
Consumes Water and
Chemicals, Legionella 7
Disadvantages
Consumption 8
Max 1% Design Wet Bulb in New Zealand + 20°C in Christchurch

6. COP variation with SCT of a commercially available semi-hermetic transcritical CO2 compressor at various SSTs operating in subcritical mode -1
C SST Vs SCT 20 -5
C SST Vs SCT
-10
C SST Vs SCT 15
Compressor 50 Hz
Speed, rpm ,450
Swept
volume, m 3 /h 10
Motor rating, kW
COP 5
Constant Suction Superheat (SSH) 5 K
Constant Liquid
SubCooling
(LSC) 3 K 6 10 14 18 22 26 30
Saturated condensing temperature with Evap. condenser temperature

7. Variation of compressor cooling COP
with compressor suction superheat at
75, 80, 90 and 100 bara discharge pressure
75 bara compressor discharge pressure
80 bara compressor discharge pressure
90 bara compressor discharge pressure
100 bara compressor discharge pressure 4
3.8
3.6
3.4
3.24
3.2
Cooling COP 3
2.8 5 10 15 20
Compressor Suction Superheat after
suction Heat Exchanger - K

8. Variation of the compressor discharge
temperatures with suction super heat at
75, 80, 90 and 100 bara
75 bara compressor discharge pressure
80 bara compressor discharge pressure
90 bara compressor discharge pressure
100 bara compressor discharge pressure
130
120
113
110
100 90 80 70
Compressor Discharge Temp., °C 5 10 15 20
Compressor Suction Superheat after
suction Heat Exchanger - K

9. Variation of the combined cooling
and heating COP with compressor suction
super heat at 75, 80, 90 and 100 bara
75 bara compressor discharge pressure
80 bara compressor discharge pressure
90 bara compressor discharge pressure
100 bara compressor discharge pressure 9
Combined Cooling & Heating COP 8
7.48 7 6 5 10 15 20
Compressor Suction Superheat after
suction Heat Exchanger - K

10. Water cooled subcritical CO2 refrigerating system with chilled water storage for milk cooling and water heating in transcritical mode

11. Summary of CO2 to water and water to milk PHX capacities and temperature
Heat exchanger entry and leaving temperature, C
PHX Function Description
Capacity
(1) CO
side
Water side 2
Water Cooling and Heating with CO kW
T in
T out
T in
T out 2 No
Function 2
(2) CO 2
condenser subcritical 21 29
(SCT) 26 23 26 8 2 CO
evaporator chilled water
18.3 26 -1 23 3 17
(2) 2
Transcritical CO
water heater 24
113 18 15 85
Milk Cooling with Water
Milk side
Water side 10
(3)
Stage 1 milk cooling with cooling tower 20
38.6 26 23 35 11
(3)
Stage 2 milk cooling with chilled water 32 26 6 3 23 12
(4)
Insulated milk storage tank cooling 1 - - 3 4
(1) Capacities are based on 8 hour compressor operations between 11pm and 7am the next day for two milking cycles daily.
(2) Compressors operate transcritically for about 6.0 hours and subcritically for 2.0 hours daily.
(3) The capacities for Stage 1 and 2 cooling are based on two daily milkings lasting two hours each
(4) The insulated milk tank wall requires continuous cooling with chilled water except when the empty tank is Cleaned In Place (CIP).

12. Calculation of Energy Consumption
Cooling tower removes 37% of kWh equating to COP = BkWh
Transcritical CO2 removes 48% of kWh equating to 103 COP = BkWh(1)
Subcritical CO2 removes 15% of kWh equating to 32.2 COP = BkWh(2)
Total daily EEC at energy consumers BkWh
Say transmission and semi-hermetic motor efficiency, %
Daily metered EEC for milk cooling and water heating, Items 4 ÷ kWh
Annual EEC occurs in about 300 days (8-10 weeks dry) 12,780 kWh
Current EEC of refrigeration and water heating - 53% of 88,620 kWh 46,969 kWh
Reduction in EEC with CO2 refrigerated chilled water and transcritical water heating 34,189 kWh
Annual EEC reduction for milk cooling and water heating, %, Items 9 ÷
Overall reduction in EEC based on total annual consumption of 88,620 kWh, % 38.6
(3)
This includes all water heating and eliminates
This includes milk storage tank cooling
Based on an evaporative −1°C SCT/16°C SCT from Slide 5

13. Calculation of Energy Cost Savings
Night tariff EEC (Item 7 from previous slide) = $1, – 12,780 $0.1328/kWh
Day tariff EEC 88,620 kWh – 46,969 kWh = $10, (Item 8 from previous slide) = 41,621 $ kWh
Total EEC cost on Day/Night tariff = $11,906.00
Present situation – all EEC on all day tariff, = $19, ,620 $0.2215
Savings with Day/Night tariff, = $ 7, Item less Item 5.3.3
Savings with Day/Night tariff, %

14. Conclusions
The use of a dual chilled water tank system – one cold and one warm – used as part of a water cooled CO2 refrigerating system with heat recovery for CIP hot water generation has the following major benefits.
The milk is cooled from 38.6°C to the final mandatory temperature of 6°C in 2 consecutive stages in about 5 minutes. Milk added at 6°C from the next milking will therefore not increase the temperature of the milk in the vat. This is far superior to the four component time temperature protocol for rates of milk temperature reductions and variations on a New Zealand dairy farm as mandated by the NZMPI.
Subject to an evaluation of the existing HFC systems, the dual tank system for cold and warm chilled water may be implemented at moderate costs to existing HFC systems. Existing HFC systems would likely be too big.
There is no need for expensive ICE build storage systems if a dual temperature chilled water tanks are used in existing systems.
All 32% of the water heating electrical energy is saved as the CO2 compressor operating in transcritical mode during water chilling heats all the water required for CIP cleaning to 85°C at a combined heating and cooling COP of about 7.5.
A maximum 38% of the heat in the milk is removed by cooling tower or evaporative condenser water.
There is a 72.2% reduction in the combined EEC for milk cooling and water heating. This equates to 38.3% of the total farm EEC.

15. Conclusions (Continued)
With a day/night tariff the cost of the EEC would reduce by about 39.3% due to a 38.6% reduction in EEC when using CO2 refrigeration with heat recovery.
New Zealand’s nearly 12,000 dairy farms collectively consume about 3% of the nation’s EEC. Rolling this technology out nationwide to all dairy farms would reduce New Zealand’s national EEC by about 1%.
Any new CO2 refrigerating system would future proof the refrigeration requirement against any detrimental effects of the HFC phasedown.
It is important to realise that the degree of Stage 1 cooling with cooling tower water needs to be limited to ensure that there is sufficient heat load available for the CO2 compressor to have sufficient heat rejection to heat an adequate quantity of water.
There are several ways in which HFC/HFC or inverted HFC/CO2 heat pumps for water heating may be incorporated into existing HFC refrigerating systems on dairy farms.

16. Acknowledgements
The author wishes to gratefully acknowledge the cooperation and assistance received from Ms Terese Connor of Genesis Energy Ltd by providing the electrical energy tariff details in five NZ dairy districts, i.e. Hamilton and surrounding areas, Cambridge – Both on the North Island, - and Nelson, Christchurch, and East and South Otago, including Dunedin. This proved invaluable in estimating an average value of current Electrical Energy Costs (EECs) and estimating reductions in Electrical Energy and consumption and EECs.