Thermal heat recovery system for the dairy shed
A thermal heat recovery system works by capturing the heat from the refrigerant gases on the milk cooling plant to heat water needed for cleaning the milk harvesting plant. This preheating of water can save energy and reduce carbon emissions.
The heat recovery system inserts between the compressor on the milk vat and the air cooled condenser to extract the heat during milk cooling. The hot refrigerant gases with high pressure from the compressor on the milk vat are transported to the heat recovery system where the heat is released into the cycling water in the system. The heat exchanger is a brazed plate heat exchanger and the number of plates will vary according to the power of the cooling unit.
There may be improved efficiency of the milk cooling plant however this benefit has not been quantified in this analysis.
Photo: Milk vat (image courtesy of Jason McAinch)
Factors to consider
These systems can by implemented on a range of milking cooling plants. However the energy saved in the system will depend on the following:
Correct installation on the existing milk cooling system to achieve the target water temperature.
The volume of the water tank and volume used during and between milkings.
The amount of milk to be cooled will determine how much heat can be captured, particularly in seasonal calving herds where there can be large changes in the quantity of milk.
The network water temperature.
Type of refrigerant gases and the configuration of the installation.
Combining an evacuated tube solar hot water system with the thermal heat recovery system may be an option on some farms to heat the water to the target temperature. This type of system may limit the need to use the electric hot water service however, complexity of combining different systems needs to be considered.
The energy savings
|Thermal heat recovery|
|Net present value (at 7% discount rate, 3% inflation on cash flows)||$16,800|
|Internal rate of return (real)||27%|
|Years to breakeven (before interest and tax)||5 years|
The thermal heat recovery system was a profitable investment. This system would recoup the initial cash outlay and will cost less than business-as-usual in year five. After year five the thermal heat recovery system would be a cheaper alternative to electric water heating in the business-as-usual option. At the end of year ten the system was $16,800 better off than business-as-usual and had an internal rate of return of 27 percent.
The capital investment of a heat recovery system for the hypothetical case study farm was $7,375 plus $3,000 for installation (TRS Australia), but can vary between $7,000 and $11,000 depending on the specific requirements of the installation and style of the compressor. The system is guaranteed to last for five years however it is expected that it will last 15 years with annual maintenance of $100.
Business as usual option
The heat recovery system will heat the 700 litres of water to 65oC required by the case study farm for sanitising the milk harvesting plant. The preheated water enters the existing electric hot water service for further heating to 90oC overnight.
The energy saved under this system would be equivalent to the energy required to heat water from 15 oC to 65oC or 41 kWh per day. This amounts to $2,095 energy saved in the first year.
Do they pay?
The water temperature needs to average at least 33oC to be better than business-as-usual, i.e. a net present value greater than zero. The thermal heat recovery system would need to have a capital cost less than $31,943 for it to remain as a better option than business-as-usual.
Any change to the preheated water temperature and capital cost will influence the profitability of this system. Regular testing and checks of the unit may be required to ensure the system is working efficiently over the duration of its life.
The reduction in carbon emissions was 18 t CO2-eq representing a 67 percent decrease in energy consumption.