Dairy Milking Pump Energy

One of the largest energy users on a dairy farm is the milk vacuum pump. This pump, run by an electric motor, pumps air out of the vacuum lines of the milking system, so that a steady amount of vacuum (usually ~11 inches of water column) exists in those vacuum lines regardless of how the milking system is being operated.

Traditionally, this was achieved by running the pump at full speed and bleeding excess air into the vacuum lines if the amount of vacuum ever got too high. This crude but effective method does a fine job of controlling vacuum level, but is not very energy efficient.

The preferred method, now, is to use something called a "Variable Speed Drive (VSD)", also called a "Variable Frequency Drive (VFD)" to control the speed of the electric motor that is running the pump, turning it up whenever more vacuum is needed and turning it down when less is required. This approach yields significant energy savings, but the cost of installing a VSD is also significant. Thus, if your farm doesn't have a VSD on the vacuum pump, it is prudent to analyze the potential savings before you make any sort of decision on whether or not to install a VSD.

While the installation cost of a VSD can be obtained from your local milking equipment contractor, the projected savings can be a bit of a challenge to calculate. Fortunately, Penn State Extension has created a Dairy Vacuum Pump VSD Calculator tool that helps take some of the complexity out of the process. Based on long-standing algorithms for energy use and savings, the calculator automatically calculates the projected energy and cost savings to be obtained from installing a VSD. If you know the installation cost of the VSD, it can even calculate the payback period for the installation.

The calculator is available on the Penn State Extension website, and comes in the form of a spreadsheet file (.xlsx). Therefore, you will need a computer with spreadsheet software installed, and should be somewhat familiar with using a spreadsheet to enter numbers. When you open the file, you will see that the first tab of the spreadsheet includes a set of instructions with an annotated example calculation to help clarify how to use the tool properly. The second tab is the actual tool.

The information needed to use the tool is as follows:

• Cost of electricity ($/kWh). Probably the simplest way to do this is to look at a recent electricity bill, find out the total amount of money you were billed, and divide this by the total kilowatt hours (kWh) of electrical energy that you used (this should also be listed on your bill). While electricity bills can be rather complicated and confusing (see Understanding and Analyzing Your Utility Bills), this simple measure of dollars divided by kilowatt hours is actually a surprisingly robust way to estimate energy costs and savings.
• Size of vacuum pump (hp). Look on the electric motor of the pump - there should be a metal nameplate with the motor's size, in horsepower, stamped or printed somewhere on that plate. This is the rated maximum rate at which work can be done by the motor that drives the pump.
• Number of Milking Units. The milking unit is the device that connects to a cow and milks it.  For example, a milking parlor that can hold 8 cows at once would count as eight milking units.
• Hours of Operation Per Day. Count up the total amount of time per day that the vacuum pump runs. This includes milking and cleaning time. For example, let's say that a farmer milks twice per day, spending three hours at each milking, plus a two hour cleaning cycle each day. This would equal six hours per day of milking plus two hours of cleaning, or eight hours per day.
• Cost to install VSD. This is an optional number that you can enter if you know the cost for installing a variable speed drive on your milking system. Keep in mind that a milking system VSD will be more expensive than merely the cost of the variable speed drive, due to the need for additional sensors, control circuitry, signal filters, and suitable housing and mounting equipment (plus installation, of course). This is an optional input - if you don't enter it, the calculator tool will still work, but with the exception that it will not be able to calculate the payback period for installing the VSD.

Once you enter these numbers into the calculator tool, the spreadsheet immediately estimates the energy use by the current milking pump, the energy use if a VSD is installed, and the expected annual energy and cost savings. The calculator is based on an assumed milking system requirement of 35cfm reserve plus 3.2 cfm/unit, a pump efficiency of 10 cfm/hp, and an operating factor of 0.90, all based on studies of milking systems (Ludington, 2004). If you also entered the installation cost, the calculator tool will give you an estimate of the payback period. This number estimates the amount of time it will take for the cost of installing the VSD to be compensated for by the energy cost savings due to reduced energy use. Generally, if a payback period is less than 5 years, it is considered to be economically attractive.

One of the newer developments in milking systems is the advent of robotic milkers - machines that attach and detach the milking units automatically rather than using human labor. So far, the energy implications of these devices has been minimal - in some cases, robotic milkers have been measured to increase energy use, whereas in other cases, they decrease energy use.  However, the magnitude of the change has not been great. Thus VSDs remain the main opportunity for reducing milking system energy use. In fact, new milking systems tend to have VSDs included as a normal component. Be sure this is the case, though, if you are building a new milking center for your barn.

Reference

Dairy Farm Energy Management Guide. By David C. Ludington, Eric L. Johnson, James A. Kowalski, Anne L. Mage. Published: 2004, Southern California Edison.