Farm Energy Bills Gotcha? What to do - Part One
By Scott Sanford, Sr. Outreach Specialist
This article was first printed in the November - December 2007 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
Many agricultural enterprises are feeling the pinch with the recent sharp increases in energy costs. Most are looking for ways to decrease energy bills. There are numerous things that can be done at the farm level to reduce overall energy consumption. This article will concentrate on conservation for electrical and natural gas or propane applications on farms and rural properties.
Energy usage in the U.S. and the world has steadily increased at an annual rate of 2%. At this rate of consumption, studies based on currently known reserves predict oil usage will peak in 20 to 40 years and then rapidly decrease as reserves are depleted. A report by the International Energy Agency states that U.S. energy consumption is currently 8000 kg per person of oil equivalent, while European countries use 45 to 50% less energy per capita. However, Wisconsin’s overall energy usage in agriculture has remained steady since 1970, except for a peak in the late 1970s into the early 1980s. Energy usage in agricultural production in the U.S. has decreased 48% per unit of output since the mid 1950s. Electrical energy usage has increased steadily since 1970 in all sectors of the economy except agriculture, where usage has remained flat.
There is hope for the energy-bill blues. There is newer technology, such as variable speed motor drives, that can reduce motor energy usage in variable load applications by 60% or more. Replacing older, less efficient equipment with newer technology can save energy but sometimes energy can be saved by simply changing management practices, such as implementing a scheduled maintenance program or using checkbook water management for determining irrigation timing. Energy-efficient equipment often has short paybacks based on the energy savings offsetting the capital cost of the high efficiency equipment. The best time to consider high-efficiency is when expanding an enterprise or when equipment has to be replaced. Energy conservation is not about making sacrifices, but is about making educated choices.
On a typical dairy farm about 70% of the electrical energy is used for collecting and cooling milk. A 2003 study of 30 New York State dairy farms indicated that vacuum pump consumed 17%, milk cooling 25%, lighting 24%, ventilation 22% and water heating 4%. All other uses, including feeding equipment and manure handling, accounted for only 8%. Water heating is misrepresented in this study because only electricity was audited and water heating is often fueled by propane, natural gas or heating oil. An energy efficient dairy would be one that uses less than 750 kWh per milking cow per year, has vacuum pump energy usage of less than 50 kWh/cow-milking-yr and milk cooling energy use of less that 0.7 kWh per hundred weight of milk. A 2003 report from the Center for Dairy Profitability indicated that the average utility energy expenses can range from $96 per cow for dairies with less than 50 milking cows to $62 for dairies milking more than 250 cows.
There are several technologies that will reduce energy usage for dairies. Refrigeration heat recovery units will capture heat from the refrigeration system that would normally be expelled to the atmosphere and transfer the heat into water. The preheated water is then routed to the water heater, reducing the heating requirements and increasing water heater capacity. Depending on the amount of hot water used and the amount of milk cooled, up to a 50% energy savings is possible.
Costs of refrigeration can be reduced with the use of a well-water cooled heat exchanger, often called a precooler. Well water is used as a coolant to cool the milk. If the water flow rate is high enough and the heat exchanger area is larger enough, it is possible to cool the milk to within 3°F of the well water temperature. The water flow versus milk flow ratio through the precooler is key to maximum cooling; a minimum of one gallon of water is need for each gallon of milk flowing through the precooler. However many precoolers are design for 2+ gallons of water per gallon of milk to achieve maximum cooling. A study in Wisconsin indicated the average precooler only has a ratio of a half of a gallon of water per gallon of milk because of limited water supplies and high milk flow rates. A variable speed milk pump can be used to slow the milk flow rate, therefore increasing the water to milk flow rate. An additional 15 to 20°F of cooling can be expected with a variable speed milk pump. Precoolers can save up to 60% in milk cooling costs.
Precoolers and refrigeration heat recovery units are competing technologies. If a precooler is used, there will be less heat available to transfer to water from the refrigerant. So which should one use? That depends on the amount of milk cooled and the amount of water heated. Generally it is more cost effective to replace water heating needs first because a standard water heater is only 50 to 55% efficient overall versus a refrigeration compressor which is about 250% efficient. Generally, dairies milking less than 100 cows will benefit from a refrigeration heat recovery unit while dairies with more than 150 cows can usually benefit from both a refrigeration heat recovery unit and a precooler.
For the last 50 years milk has been cooled on farms using reciprocating refrigeration compressors that work much like a gasoline engine. These have many moving parts. A new design that was introduced to the agricultural market about 10 years ago is called a scroll compressor. These are about 15 to 20% more energy efficient and only have one moving part, an oscillating scroll. The oscillating scroll moves within a fixed scroll and forms cavities that get progressively smaller as the scroll rotates. On most dairies it is not economical to replace a working refrigeration compressor, but planning to replace a reciprocating compressor with a scroll in the event of a failure is an opportunity to increase your energy efficiency. The cost of a scroll compressor is only about $50 to $100 more than a similar sized reciprocating compressor, but typically new mounting holes and some re-wiring of controls will be needed which can increase the incremental replacement cost by $300 to $500. Scroll compressors can also be used on walk-in coolers and household air conditioning systems.
The Importance of Maintenance
Refrigeration system maintenance is important for energy efficiency. Dirty condensing units can rob your system’s ability to expel heat, which increases system pressure and energy use. A study at the University of Wisconsin found a 3 to 5% reduction in energy usage after cleaning dirty condensing coils. These should be cleaned with a special degreaser/detergent that won’t corrode the copper and aluminum bonds that are important for dissipating heat. The condenser / evaporator cleaner can be purchased from refrigeration equipment suppliers for those who wish to do their own cleaning. The electricity must be disconnected and any open motors or electrical boxes protected from water spray before starting to clean. Sometimes removal of the fan motors to gain access to the back of the condenser can help facilitate cleaning. The degreaser is mixed with water and then sprayed onto the condensing unit fins, allowed to soak for 5 to 10 minutes and then rinsed off. The process should be repeated for very dirty units. The fins should also be straightened if bent to allow free flow of air. A fin comb is used for straightening bent fins. It is highly recommended that your refrigeration service provider check the refrigerant level and pressures annually to detect any leaks or other problems. Condensing units should be located in areas with clean, free flowing air. In the winter the expelled heat can be diverted for space heating, but in the summer the heat should be diverted directly outside.
Variable Vacuum Pumps
Variable speed controls have been used for years in many industrial settings, but were too expensive to justify until cost of these units decreased with the computer age. Variable speed vacuum pumps were first introduced about 1996 and have been shown to reduce energy costs by an average of 60%. A traditional vacuum pump has a pneumatic regulator that admits air into the vacuum system to regulate the vacuum level while a variable speed vacuum pump uses a pressure sensor to detect vacuum level and then adjust the motor speed to maintain the vacuum level set point. These systems are as accurate as or more accurate, than pneumatic type regulators. A rule of thumb: if you are milking 3 times daily, or about 6 to 8 hours per day, it will likely be economical to install a variable speed control on your vacuum pump. A variable speed control can be installed on a rotary vane vacuum pump, but blower type vacuum pumps are preferred because the pump can operate at lower speeds without damaging the pump. Dairies that are currently using two vacuum pumps can often milk with just one vacuum pump outfitted with a variable speed controller.
All ventilation fans are not all created equal! There is independent testing of ventilation fans for energy consumption and performance by BESS Lab of the University of Illinois (www.bess.uiuc.edu) and the Air Movement and Control Association (www.amca.org). Both organizations publish their test data and have it available for free on the web. The difference between the worst performing fans and the best is a factor of two when considering the energy efficiency measured as air flow per energy unit consumed or cfm (cubic feet per minute)/watt. Generally, larger diameter fans and fans with diffuser cones are more energy efficient. Maintenance of fans is also very important. Dirty louvers that don’t open freely can reduce air flow by 40%, and loose fan belts can reduce air flow by 30%. When purchasing new fans, look for manufacturers that have high efficiency ratings (larger number is better) and offer self-tensioning devices for belts.
Very few people think about the energy efficiency of water heaters, but these units can be very inefficient, especially gas and oil fired units. Electric units are considered 99% efficient at transferring heat, while most standard gas or oil fired units have an 80% thermal efficiency. But that is only half the story. Water heaters can have very high standby losses: heat lost through the tank walls to the surroundings. This can range from about 0.5% per hour for highly insulated water heaters to greater than 3% per hour. The average water heater has about 2 to 2.5% heat loss per hour, which equates to 50 to 60% standby loss per day. This reduces the overall efficiency of the average water heater to 50 to 55%. High efficiency condensing-type water heaters are available that have thermal efficiencies of 94% and standby losses of 1% or less per hour, increasing the overall energy efficiency to about 75%. Electric water heaters are generally low standby losses because they are well insulated, but can cost more to operate depending on your electrical rate. The heating rate of electric water heaters is generally less (gallons heated per hour) than gas or oil fired water heaters, so in some cases they can’t keep up with the demand.
This is the first in a two part article by Scott Sanford, Senior Outreach Specialist at University of Wisconsin Department of Biological Systems Engineering. Part Two talks about conserving through lighting technology, use of livestock waterers, alternative fuels, irrigation and grain drying.
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