University of Minnesota Guidebook on Optimizing Energy Systems for Midwest Dairy Production

Transcription

1 University of Minnesota Guidebook on Optimizing Energy Systems for Midwest Dairy Production February 2017 Edition Prepared by the Renewable Energy Team at the West Central Research and Outreach Center Project funding provided by the University of Minnesota Initiative for Renewable Energy and the Environment (IREE)

2 Table of Contents Introduction... 1 The WCROC Dairy... 1 WCROC Dairy Milk Production... 2 WCROC Dairy Energy Usage... 4 Dairy Energy Efficiency Recommendations... 8 Lighting... 8 Water Heating... 2 Variable Speed Drives... 4 Refrigeration Compressors... 5 Refrigeration Heat Recovery... 5 Plate Cooler... 6 Renewable Energy... 7 Solar Energy... 8 Solar Thermal... 9 Solar Electric Wind i

3 Introduction The agricultural industry consumes an immense amount of fossil-fuel in the production of food, feed, fiber, and energy. From the electricity that cools milk, to the fuel that is burned in combines and tractors in grain fields, to the trucks that bring goods to market, and to the nitrogen fertilizer that nourishes plants; the agricultural industry is captive to large and constant supplies of a wide range of fossil energy. Agriculture s dependence and thirst for fossil-fuel carries significant economic, environmental, and social risks for the nation and world. Through past investments and institutional experience in renewable energy and energy efficient research, the University of Minnesota has a globally unique opportunity to lead a new green revolution - a revolution that greens energy currently consumed within the agricultural industry. According to the U.S. Department of Agriculture s National Agricultural Statistics Service, Minnesota was the 7 th ranked state by milk production in 2015 producing about 9.5 billion pounds of milk with about 460,000 cows. Clearly, dairy farms are a large and important industry in Minnesota. Harvesting milk is an energy intensive activity making dairy farms a great place to improve energy efficiency, reduce dependence on fossil fuels, and potentially save money for the producer. The West Central Research and Outreach Center (WCROC) has undertaken the task of measuring energy usage in the dairy barn (milking parlor) with the goal of providing data to help develop recommendations for more efficient dairy energy systems. The dairy at the WCROC is a grazing dairy with cows spending the entire year outside. Producers assessing operational changes based on data presented here should carefully consider the affect differing management methods might have on expected results. The WCROC Dairy The WCROC dairy milks between 200 and 280 cows twice daily and is representative of a mid-size Minnesota dairy farm. The cows are split almost evenly between conventional and certified organic grazing herds and all cows spend the winter outside in confinement lots near the milking parlor. The WCROC dairy provides an ideal testing opportunity to evaluate and demonstrate the effect of on-site renewable energy generation and energy efficient upgrades on fossil fuel consumption and greenhouse gas emissions. The existing dairy equipment is typical for similarly sized dairy farms and included none of the commonly recommended energy efficiency enhancements such as a plate cooler, refrigeration heat recovery, or variable frequency drives (VFD) for pump motors when this project started. The WCROC dairy barn was originally put into service in 1972 with a capacity of 60 tie stalls and space for maternity pens, calves, and young stock. When the dairy operation converted to grazing in the 1990 s, part of the barn was reconfigured to include a swing nine milking parlor. The rest of the barn including the 60 tie stalls is currently unused and not directly heated, but is still ventilated with fans in the summer. Figure 1 shows the barn configuration and dimensions of the actively used space. 1

4 There are four general categories of energy usage in the WCROC dairy operation: thermal energy in the milking parlor provided by natural gas, Electrical energy in the milking parlor, water used every day in the milking parlor primarily for cleaning, and fuel consumed by farm vehicles and tractors. Water is also consumed by the cows from pasture water stations, but cow drinking water is not considered in this guide. Dairy Barn Configuration WCROC Dairy Milk Production Milk production and energy consumption data for the WCROC dairy parlor has been tracked since 2013 through 2016 with goal of better understanding where energy is used. It is hoped that this understanding can help producers make their operations less energy intensive and more economically efficient at the same time. The overall dairy herd size and milk production over the monitoring period including relative contributions from the conventional and organic herds are shown below. Year ave WCROC Dairy Production Cows Total Milk (lbs) Total Milk (gal) Ave. Milk (lbs/day) Ave. Milk (gal/day) Conventional 115 1,975, , Organic , , Total 207 2,929, , Organic cows in the WCROC dairy do not produce as much milk per cow as the conventional cows. This difference is the result of cow management decisions which are based on the high price of certified organic feed. Certified organic cows are fed or graze a ration higher in forage and fiber so less digestible nutrients are available to achieve maximum milk production. 2

5 The growth in herd size over the monitoring period with seasonal variation, and the variation in milk production over a single year is shown in the following two graphs. Production drops in the fall and spring when more cows are dried off. The period of time from May of 2015 through April of 2016 is important because that period produced the most complete set of energy use data which will be used to make specific recommendations. 3

6 WCROC Dairy Energy Usage The WCROC dairy barn has a dedicated utility meter for both natural gas and electricity consumption which were used to monitor total barn energy usage. In addition a data logger was installed in late 2013 to monitor individual electric loads and water usage. The overall results are shown in the following graph. Over the 3 plus years of data monitoring from late 2013 through 2016, the WCROC dairy produced about 340,000 gallons of milk per year milking an average of 207 cows per day. Collecting and cooling the milk, as well as cleaning the parlor and milking equipment, consumed an average of about 1.4 gallons of water and 2.8 megajoules of energy per gallon of milk. Annual data for each year is shown below on a per cow and per gallon of milk basis. 4

7 The following table summarizes energy inputs for the WCROC dairy over the monitoring period. WCROC Milking Parlor Energy Usage Energy Usage Annual Ave. Ave./cow/ day Ave./gal milk Natural Gas (MJ) 502, Electricity (kwh) 111, Total Energy (MJ) 936, Total Water (gal) 478, In order to make recommendations for better energy efficiency, it is important to understand where energy is being used and the relative size of each load. The following pie charts reveal the best places to reduce energy use. The WCROC dairy uses a little more than 1300 gallons of water every day (not including cow drinking water) of which about 560 gallons is hot water. Almost all of the water used in the parlor is for clean-up tasks which are typically not given a lot of consideration beyond the fact that they are necessary. However, at 1.5 gallons of water per gallon of milk, cleaning up is a significant investment in milk production and is likely to become more important as competition for potable water resources increases in the near future. Water usage at the WCROC dairy does not include a well water cooled plate cooler which might increase water consumption if the warmed well water is not collected and used for cleaning tasks or watering cows. The following charts detail how electricity is used in the WCROC dairy parlor. 5

8 Milk cooling uses the largest share of the electric load, but ventilation is almost as large in the summertime (25%) and electric heaters become more significant in the wintertime (12%). The electric heater load is from 2 small (1500Watt) milk house heaters deployed in the utility room to prevent water lines from freezing. The WCROC parlor does employ a variable frequency drive (VFD) on the vacuum pump which decreases the milk collection portion of the electric load. More details on VFD s will be included in a separate section. The following graph shows how the individual electric loads vary over a full season in the WCROC dairy. The two bulk tanks at the WCROC dairy (one for conventional milk and one for organic milk) use different refrigeration compressor technologies. The conventional tank uses an older reciprocating compressor and the organic tank uses a newer scroll compressor. This technology will be discussed in more detail in a later section. 6

9 The WCROC dairy parlor uses a natural gas water heater and a natural gas furnace to heat the parlor, and diesel fuel is used in the pressure washer to heat wash down water. Adding these energy loads results in the following charts showing the total energy usage in the parlor. With all energy sources graphed it is easy to see that parlor heating is the dominate load and that water heating is the next largest becoming the largest in summertime. It is also interesting that the diesel fuel used in the pressure washer only 5 gallons at a time is a pretty significant load, on par with milk collection or the electric heaters. Almost twice as much total energy is used on a daily basis in January as is used in July due to parlor heating, including electric heaters. The current parlor at the WCROC is an old tie stall barn built in the 1970 s with little insulation and the assumption that the housed cows would provide significant heat. Moreover, the furnace is configured so that all air brought into the barn is fresh air from outside. This means cold outside air is continually being heated in the winter and exhausted through the ventilation fans and porous building envelope one of the least efficient ways to heat a building. If designing a new milking parlor, more in-depth consideration of the heating and ventilation systems will be time well spent. The final category of energy usage tracked at the WCROC dairy was vehicle and tractor fuel usage. Over the monitoring period, an average of 522 gallons of gasoline and 1310 gallons of diesel were used annually. In energy units, the combined average fuel use is about 241,000 megajoules annually. These fuel quantities do not include tractor field work. Vehicle fuel use data was not combined into the pie 7

10 charts for total energy use in the parlor because most vehicle use does not strictly apply to the milking parlor, but it is interesting to note that the size of the vehicle energy load would be about 25% of the total dairy average daily energy load. This means the amount of energy used to fuel farm vehicles is similar to the amount used to provide hot water in the parlor. Dairy Energy Efficiency Recommendations According to a dairy energy efficiency report prepared for the Minnesota Department of Commerce 1, dairy farms in the U.S. consumed between 400 and 1,700 kwh per cow annually, or $0.035 to $0.045 per hundredweight (cwt) of milk produced in The WCROC dairy by comparison used about 540 kwh per cow per year, costing about $0.038 per cwt of milk at the low end of the spectrum. The WCROC dairy is a 100% grazing dairy and uses natural gas for parlor and water heating explaining why it is not surprising that electricity costs are relatively low when compared to all dairies. The Wisconsin Department of Trade and consumer Protection published a dairy energy handbook 2 reporting that 25% of Wisconsin dairy farm energy use was attributed to milk cooling, 19% to ventilation, 18% to water heating, 17% to milk collection, 15% to lighting, and 6% to electric space heaters in This agrees pretty well with the measured results shown above for the WCROC dairy ignoring parlor heating, which wasn t considered in the Wisconsin study, and assuming water heating is done with electricity instead of natural gas. The only real difference is the larger proportion of electricity used for lighting in the Wisconsin data. The WCROC parlor is not used to house cows and uses fairly efficient fluorescent bulbs. The Minnesota Department of Commerce report cited above documented the results of energy audits and efficiency recommendations on 30 dairy farms of varying sizes (40 to 500 cows) in the Hastings Cooperative Creamery Company. Of the 30 farms, more efficient lighting was recommended for 29 of them with an average estimated simple payback of 3.3 years. High efficiency electric water heaters were recommended to 19 farms with an average estimated simple payback of 6.2 years. Variable speed drives (VFD) were recommended to 18 farms for their vacuum pump and to 10 farms for their receiver jar pump with average estimated simple paybacks of 14.2 and 6.6 years, respectively. Scroll compressors were recommended to 11 farms with average estimated simple payback of 24.7 years. Refrigeration heat recovery had an average estimated simple payback of 7.8 years for the 11 farms receiving the recommendation. Ventilation upgrades and plate coolers were each recommended to only 4 farms with average estimated simple paybacks of 6.5 and 9.5 years, respectively. The low number of plate cooler recommendations was primarily due to the fact that most farms already had them. Lighting Lighting is the most often recommended efficiency upgrade to dairy farms due to the fairly simple and unintrusive nature of the replacement. If fluorescent fixtures already exist, they can sometimes be converted to a more efficient bulb type. There is also a direct LED bulb replacement for fluorescent fixtures. Replacing the entire fixture is the most efficient because the fluorescent ballast still uses excess energy even with LED bulb replacements. 8

11 LED lighting is the most efficient lighting currently available and has come down greatly in price over the last 5 years. Your dairy equipment provider probably can offer a lighting analysis and an LED light product. Also, your electrician can probably give you a quote on a lighting upgrade as new lighting products are no longer a specialty niche as they were even just a few years ago. The Clean Energy Resource Teams (CERTs) is a statewide partnership with a mission to connect people with resources to facilitate clean energy and energy efficiency projects. CERTs has an extensive web site 3 with a lot of information about lighting options for anyone wanting to learn more. The U.S. Department of Energy also has a lot of information 4 about lighting especially larger output types that are likely to be found in older dairy barns. There is a lot of variability in lighting efficiency even within the same technology, like fluorescent bulbs, so an assessment by a good contractor is probably your best bet for an accurate savings estimate. A simple way to estimate potential energy and cost savings for a lighting upgrade is to look at your average electric bill and multiply your total electrical usage by a percentage between 6 and 15% depending on whether your operation is more like the WCROC operation (fluorescent fixtures and cows outdoors) or more like an average operation. Now having an estimate of how much electricity you are using on lighting, you can contact your equipment supplier or electrician for an estimate of replacement costs and expected savings. Lighting upgrades has proven to be the energy efficient upgrade with the quickest payback. Also, converting to LED lighting can be expected to reduce maintenance replacing bulbs due to their longer life expectancy and possibly better lighting conditions. Water Heating As was shown above, water heating accounts for about a fourth of the total energy used in a dairy parlor. In the WCROC dairy this amounts to about 2.7 gallons of hot water per cow per day. Almost all of this water is used to clean the parlor and milking equipment. Some economies of scale can be realized in that the amount of water needed for cleaning is not very dependent on the number of cows milked so milking more cows will generally decrease water usage per unit of milk. Most farms are equipped with a conventional storage water heater that keeps a fixed volume of water, typically 100 gallons, at a set temperature, typically 170 to 180 F. Storage water heaters suffer from stand-by losses which occur as the storage volume loses heat when not in use like overnight. These losses can be minimized by buying a better insulated water heater or adding after-market insulation. Another way to improve efficiency of heating water is with an electric water heater. Electric water heaters typically have an efficiency rating of 90 to 95% because the heating element is immersed in the water so very little energy is lost. Fuel burning water heaters have an additional source of heat loss since some of the heat produced by burning a fuel will be lost in the vented flue gasses. Standard natural gas or propane water heaters are around 60% efficient so switching to an electric water heater improves energy efficiency, but it may not cost less depending on the price of electricity relative to the price of the replaced fuel. Other water heating options include demand, or tankless, water heaters which do not have a storage tank so they do not suffer stand-by losses. A tankless water heater rapidly heats water when a tap is 2

12 opened and continues to heat until the tap is shut. The main advantage of a tankless water heater is that it never runs out of hot water which would be very useful in a dairy due to the large daily demand for hot water and the relatively long time it can take for a storage style heater to recover if it runs out of hot water. The main disadvantage is in providing a sufficient flow of hot water. A higher temperature rise needed in the heater results in a smaller flow rate of hot water. Since most farms are using well water, the incoming temperature is 40 to 50 F depending on the time of year so the temperature rise to get to 170 F is extreme. A natural gas or propane unit may be the best choice for a demand style heater if it can meet the necessary flow rate since a similar electric unit would be a very large electric load probably requiring an upgraded barn electrical service making it less economical. A further consideration with demand style heaters is not using other hot water loads while the sanitizing wash tank is filling. The wash tank is typically the most important hot water chore and requires the highest temperature water. A tankless water heater might be a good option if paired with a thermal storage tank that can preheat water prior to entering the tankless water heater reducing the required temperature rise. A system like this is currently being tested in the dairy parlor at the WCROC where a large storage tank is used to store heat collected from a milk plate cooler and solar thermal collectors. Heat stored in the tank will be used to preheat water prior to entering a tankless electric water heater. Look for future results from this research on the WCROC web site 5. The following graph shows the prominent hot water loads during a typical day in the WCROC dairy parlor and their respective flow rates. Milkers make some effort to avoid running the washing machine while the wash sink is filling and, currently, the pressure washer heats its own water with diesel fuel. There are other types of water heaters including heat pump water heaters and solar water heaters. The U.S. Department of Energy has a web site with lots of additional information about water heating 6. 3

13 Variable Speed Drives Variable frequency drives (VFD) vary the frequency of the AC current going to a 3 phase motor which changes the speed of the motor. A motor without a VFD runs at full speed anytime it is switched on, but a VFD system uses a sensor to measure something like flow rate or vacuum level and adjusts the motor speed to match the desired set point of the measured variable. Since most motors are oversized to meet the maximum possible load, a lot of energy can be saved with a VFD by matching the motor speed to the actual load. In a dairy parlor, the vacuum pump and receiver jar motors are the best options for a variable speed drive. The longer a motor runs per day, the faster a VFD will pay off. In September of 2013 a VFD was installed on the vacuum pump in the WCROC dairy parlor. The following graph shows the measured effect the VFD had on the motor s electricity usage. Milk production is also shown so it is clear the reduced electricity usage is not due to any changes in operation. The energy required by the pump to do the same work dropped over 75%, saving around $4 a day in electricity. This would lead to a simple payback in around 2.5 years a clear win for the producer. It should be noted that the VFD did fail after about 3 years and a replacement was necessary. Producers should ensure their chosen contractor has experience setting up VFD s and the device itself has a good warranty. The original VFD in the WCROC dairy may have been undersized for the 10 horsepower motor leading to its early failure. The VFD was installed next to the vacuum pump near the bulk tank 4

14 refrigeration compressors which created an elevated temperature environment that also may have led to the early failure. Honeywell has a free downloadable tool for pricing VFDs 7 that can be used to get a price estimate based on motor parameters. A VFD may be used on a receiver jar pump motor to save energy, but it is necessary to smooth and slow milk flow to the bulk tank when using a plate cooler to pre-chill milk. Keeping milk in the plate cooler longer by slowing the flow allows more heat to be transferred to the cold side of the plate cooler making it more efficient. Refrigeration Compressors Refrigeration compressors are of two basic types: older reciprocating compressors, and newer scroll compressors. In the WCROC dairy parlor, the conventional tank uses an older reciprocating compressor while the organic tank has a newer scroll compressor. The conventional dairy herd at the WCROC accounts for about twice as much milk production as the organic herd so to get an accurate assessment of the different compressors the electrical loads need to be normalized to milk production (heat load on the compressor). Dividing the average daily compressor load by the average daily milk production gives an estimate of compressor efficiency in energy units (kwh) per hundredweight (cwt) of milk. Averaging these values for each month yields a reciprocating compressor efficiency of 1.07 kwh/cwt, and a scroll compressor efficiency of 0.68 kwh/cwt a 36% improvement! The monthly variation in compressor efficiency is shown in the adjacent graph. Upgrading a reciprocating compressor to a scroll model also appears to be a good investment especially if a bulk tank is being added or upgraded. A producer considering a compressor upgrade could use the efficiency numbers above combined with the producer s milk production and electricity price to estimate potential savings. Refrigeration Heat Recovery Milk is typically cooled inside a bulk storage tank using a direct expansion (DX) chiller where heat from milk is absorbed by a refrigerant. The absorbed heat is then rejected outside via an air cooled condenser. In other words, heat in the milk is undesirable and electricity is consumed (chiller compressor and fan) to reject the heat outside as a waste product. 5

15 A refrigeration heat recovery (RHR) system harvests heat absorbed into the refrigerant and uses it to pre-heat water for cleaning tasks. The heat recovery is accomplished by inserting a water tank before the chiller s condensing unit. The tank provides tubing around the circumference of the tank (inside the insulation) for hot refrigerant vapor to pass so heat in the refrigerant is absorbed by water in the tank raising its temperature while cooling and condensing the refrigerant. There are several commercially available RHR tanks designed specifically for this purpose. These systems essentially provide a water cooled condenser ahead of the normal air cooled condenser with the added benefit that a water cooled condenser will make the chilling process more efficient. Pre-heated water from a RHR system is piped to a water heater to bring it to the final use temperature. Manufacturer claims of potential energy savings vary a great deal, but there is some consensus that a dairy operation needs to be milking between 100 and 150 cows to make a RHR system economical. Plate Cooler Plate cooler is a common term used to describe a plate and frame heat exchanger used in a dairy operation to cool milk by pumping the warm milk through one side and pumping well water through the other side. The heat exchanger, or plate cooler, must be made from stainless steel and meet FDA 3A and CIP (clean in place) standards to pass muster with the state milk inspector. Plate coolers are usually paired with a variable speed drive for the receiver jar pump to slow the flow of milk through the cooler enhancing heat transfer. A well designed system should be able to get the milk temperature to within 5 to 10 F of the well-water temperature greatly reducing the load on bulk tank compressors. Reducing the compressor load saves electrical energy and extends its life. To get optimum performance from a plate cooler, water flow should be 2 to 3 times that of the milk flow. Using that much water in a plate cooler would require dumping water into the waste stream or developing a new use for the water. For example, the WCROC dairy operation would have 2 to 3 thousand gallons of well-water each day which would potentially go into the waste water stream. In an operation housing the cows, this water could be used effectively by providing drinking water for the cows. Other operations could collect it in a storage tank and use it for cleaning; however, the amount of water collected at the WCROC dairy would be 2 to 3 times more water than is currently used for cleaning. A plate cooler may not be the best option for a grazing dairy due to the issue of excess water collection, especially as potable water becomes a more important resource in the future. Another consideration with a plate cooler is whether or not to install one with refrigeration heat recovery since both systems reduce energy used by the milk cooling system. In other words a plate cooler reduces the thermal load on the milk cooling compressor, but it is exactly that compressor thermal load that a RHR system harvests to pre-heat water. In general, a milk chiller system will be more efficient than a water heater because the chiller is moving heat as opposed to creating heat. So installing a plate cooler reduces energy use by the milk chiller while installing an RHR system primarily reduces energy used by the less efficient water heating system. Large dairies can make good use of both 6

16 systems, but smaller dairies would be advised to have an on-site energy audit performed before investing in both technologies. Renewable Energy Renewable energy includes any energy source that is replenished at least as fast as it is used. Common examples used on farms include biomass energy which uses plant material to create liquid or gaseous fuels to burn in vehicles or boilers; wind power which uses wind to turn a turbine generating electricity; and sunlight which can be collected as heat or converted directly into electricity. Much more detailed information about renewable energy systems is available in a guidebook on small-scale systems prepared by the University of Minnesota West Central Research and Outreach Center 8. As concerns about rising fossil fuel prices, energy security, and climate change increase, renewable energy can play an important role in producing local, clean, and unlimited energy to supply Minnesota s and the nation s increasing demand for electricity, heat, and transportation fuel. Renewable energy sources address economic concerns of future fossil fuel prices by offering low to zero ongoing fuel costs especially when used to generate electricity. Of course, capital costs associated with renewable energy systems can be higher than traditional sources, but the costs are decreasing with increased manufacturing volume and advancements in technology. Eliminating the uncertainties of fossil fuel supply and demand addresses one aspect of energy security by offering facility managers long term price stability and predictability in the budgeting process. It is generally best to tackle energy efficiency projects prior to installing renewable energy generation systems since improving efficiency can decrease the needed size, and; therefore, the associated costs of any generation systems. There are a wide and changing variety of incentives available for renewable energy systems and efficiency improvements. A group at North Carolina State University funded by the U.S. Department of Energy maintains a database of incentives on their web site 9 which is the best place to check what is currently available in your area. An experienced installer with NABCEP certification should be consulted early when considering a renewable energy system. A qualified contractor can provide a site assessment, cost estimate, production estimate, and will usually be knowledgeable about available incentives as well as able to help apply for them. Be sure to ask for references and to talk with previous customers who have installed similar systems. The renewable energy market has matured a lot in the last decade and most current product suppliers offer robust systems with proven track records. It is not advisable, however, for a commercial farm to be the test site for a radically new technology or a contractor s first foray into renewable energy systems. Also, utilities should be consulted early in the process as they may be able to provide advice and perhaps incentives for renewable energy systems (and energy efficiency upgrades). Some utilities may discourage on-farm renewable energy systems. This should not necessarily stop a project but may be a sign that additional investigation is warranted. 7

17 Solar Energy Solar energy can be harnessed in two ways; it can be collected as thermal energy or electrical energy. Both forms can be useful in a dairy operation due to the daily need for large and consistent amounts of hot water and electricity. Both types of solar energy system will require some kind of energy storage to make use of energy collected at times it is not being used in the parlor. For hot water systems, this is usually accomplished with a tank to store hot water. For electric systems, this can be done with batteries, but it is much more economical at this time to interconnect a solar PV system with the local electric grid and send excess power to the local utility. Battery technology is rapidly improving and utility pricing may someday make battery storage more financially attractive to producers. Solar thermal collectors (hot water) and solar photovoltaic (PV) panels (electricity) can be ground mounted or installed on a roof. Both panel types need to face south and be mounted at an angle about equal to the site s latitude (45 for most of MN) for maximum energy collection. Replacing roofing material would require the removal of any roof mounted solar panels so roofs in poor condition should be replaced prior to installing any solar panels to avoid unnecessary additional costs later. Mounting panels on the roof may require an engineering assessment of the roof to ensure it can withstand any additional wind (and possible snow) loads. Wind loading considerations often lead to panels being installed at the roof angle which is unlikely to be the best mounting angle. Mounting panels on a roof does have a few advantages; namely, not requiring additional space that might be needed for other purposes and typically having better access to the sun with fewer shading issues. Roof mounted systems may also catch snow and this could lead to uneven weight distribution and possibly roof collapse. Ground mounted systems can be installed in the optimal configuration and they can be installed high enough off the ground to allow the space under panels to still be useful as shown below. Flat Plate Solar Thermal Collectors Solar PV Panels 8

18 Solar Thermal Solar thermal energy can be used to offset about two-thirds of the energy needed for hot water in Minnesota. System sizing is best done by a qualified contractor. A few things to consider include collector location and type, freeze protection, hot water storage, and maintenance. Solar thermal collectors are widely available in two different types: flat plate (FP) collectors and evacuated tube (ET) collectors. Flat plate collectors are essentially a mini greenhouse consisting of an insulated box with an absorber and a glass or plastic cover. They are simple, robust, and largely unchanged from the earliest designs except for improvements in manufacturing processes. Six flat plate collectors were installed at the WCROC dairy in 2016 as shown in the leftmost photo above. The evacuated tube collector is a relatively new design. It was developed at a university in China in the early 1980 s with initial manufacturing starting in The design uses two nested glass tubes with air removed from the space between them creating a vacuum. The much better insulating properties of a vacuum allow the ET collectors to reach much higher temperatures than FP collectors and operate more efficiently at those temperatures. More than half the solar thermal systems in the world and more than 80% of new systems use evacuated tubes, but they are still rare in the U.S. Both collector systems are similar in cost and plumbing requirements for both systems are identical. While the glass used to make ET collectors is designed to withstand one inch diameter hail stones it is not as robust as the glass in FP collectors which might be a drawback if the collectors are located where they might be struck during farm operations. All thermal systems in Minnesota require protection from freezing temperatures. This is usually accomplished by mixing antifreeze, typically propylene glycol, with water in the system. Another method is called drain back and it involves making sure all piping is sloped such that when the system pump is turned off all fluid drains from the collectors into a drain back tank. This set-up protects from freezing by keeping all fluid in a temperature controlled space when the pump is off. A drain back system has the further advantage that the system can also be protected from overheating temperatures by turning the pump off. This might be necessary if a system component fails and repairs are needed. In Minnesota, it is not uncommon to use both a drain back system and antifreeze. A storage system is useful anytime an energy source and its intended load are not in sync. This is certainly true with solar energy and a dairy parlor s hot water load. An insulated hot water storage tank can collect thermal energy from solar collectors and store it until it is needed in the parlor. If antifreeze is used for freeze protection, some means of heat transfer will be needed since the fluid passing through the collectors cannot be the same fluid used in the parlor. There are several commercially available storage tanks designed specifically for this purpose. 9

19 Some systems use the water heater as the storage tank by adding a coiled tube into the water heater that allows fluid from the solar collectors to pass through heating water in the tank without mixing. This system has the advantage of simplicity and only requiring a single water tank, but has the disadvantage of only being able to collect solar energy when the solar collectors are producing heated fluid hotter than the water heater set point. The relatively high water heater set point in a dairy parlor would greatly reduce the efficiency of a solar thermal system since there would only be a few hours a day in the summer that the system could collect energy. A better solution for dairies is a water storage tank separate from the water heater. In a two tank system, the heat exchanger (coiled tube) is placed in the solar storage tank and well water is pre-heated as it passes through it prior to entering the water heater. This set-up allows a solar thermal system to collect heat anytime the solar fluid is hotter than the solar storage tank which is only heated by the sun. As long as the solar tank is hotter than the incoming well water temperature, the system will be saving energy by pre-heating the water entering the water heater reducing the load on the water heater. The heat exchanger can also be installed external to the storage tank, like a plate cooler. An external heat exchanger is almost as efficient as an internal one and greatly decreases the volume (and cost) of antifreeze needed as well as the complexity (and cost) of the storage tank. Maintenance of a solar thermal system is similar to any plumbing system including tanks and pumps with the added complexity of antifreeze. Antifreeze needs to be checked annually to ensure proper freeze protection. This is done with a syringe type tester very similar to that used to test automotive antifreeze. If antifreeze overheats, it can become corrosive to system piping so system temperatures must be monitored especially if not using a drain back configuration. Antifreeze also has a useful life and must be flushed and replaced about every 5 years. A solar thermal system also depends on temperature monitoring including temperature sensors and a controller so sensors will need to be checked and replaced periodically. Of course any plumbing system with additional pumps, like a solar thermal system, is likely to require additional monitoring and maintenance. There are incentives available for solar thermal systems, but certified collectors are usually required to get them. The Solar Rating and Certification Corporation (SRCC) performs testing on all solar thermal collectors using a standard protocol and provides certification that a collector meets acceptable performance standards. A collector typically needs to have SRCC certification to be eligible for federal and state incentive programs. The SRCC determines the daily energy production for each panel it tests for various sunlight conditions and fluid to air temperature differences, and reports the results on its web site

20 Solar Electric A solar PV system will generate electricity whenever the sun is shining, but will produce only low levels of electricity in low light conditions and no electricity when it is dark. This generation pattern will usually not completely coincide with when the electricity is needed on a dairy farm so; as with solar thermal, some type of storage system is necessary. One way to do this is to connect the system to the electric utility grid selling electricity to the utility when an excess is being generated and buying electricity when more is needed. This arrangement with a utility company is called net metering. Another way would be to use batteries, but this is usually only done where utility grid interconnection is not available and; therefore, would probably not be economical for the typical dairy at this time. Minnesota has a net metering policy requiring electric utilities to purchase consumer produced electricity. A utility in Minnesota must pay the retail rate for any system not exceeding 40kW in size. For comparison, a 7kW system would generate about 100% of the electricity used annually in the average Minnesota home. Net metering requires a meter that can measure electricity moving in both directions. Many currently installed meters can already do this, but if not, a new meter is usually not a large expense ( $300 to $500). Net metering policies can change so one should verify rates with the appropriate State or utility representative. Solar PV panels generate DC, or direct current, electricity which has a positive and negative side like a battery so current only flows in one direction. The electricity supplied by an electric utility is AC, or alternating current, meaning the direction of current flow reverses 60 times per second (60 Hz) in the United States. An electrical device, called an inverter, is needed to convert the DC electricity from a solar panel into the AC electricity used by most household appliances and the electric grid. A power inverter used in a solar PV system must be sized to handle the maximum voltage and current that could be produced by that system, and must be able to condition the power so that it matches the quality of power on the grid. A grid-tied inverter must also have the ability to automatically shut off the power flow to the grid in the event of a power outage. If the utility shuts down power to part of the grid for maintenance or repairs, power lines could be unknowingly energized by a consumer s power generation system posing a safety risk for line workers. Power inverters having UL listing 1741 meet these requirements and are standard in the solar industry today. 11

21 The normal inverter warranty period is 10 to 15 years, with some stretching or extendable to 25 years. Solar PV panels generally have a 25 year warranty, and modern panels can be expected to perform for 50 years or longer under normal conditions. This means the inverter will probably have to be replaced over the lifetime of the system. Inverter replacement costs should be included in any financial analysis. In summary, a grid-tied PV system consists of solar panels, a power inverter(s), a circuit breaker panel, a two way electric meter, and, of course, the utility electric grid. The panels generate DC electricity when the sun is shining which is converted to AC electricity by the power inverter. The AC electricity is distributed to the electrical load through the circuit breaker panel. When more electricity is being generated than is being used, excess electricity is directed through the meter and to the electric grid. When not enough electricity is being generated by the panels, additional power is drawn from the grid through the meter and into the circuit breaker panel. The power company keeps track of the net use of electricity and adjusts the electric bill accordingly. Many factors go into determining the performance of a solar PV system, but a simple estimate of annual electricity production from a non-tracking solar array can be made based on the nameplate power rating of the system. Analyses of data from solar arrays installed at the WCROC have determined that taking the rated DC power of a system in Watts and multiplying it by 1.3 will give a reasonable estimate of the number of kilowatt hours that will be produced by that system in a year. For example: a 10 kw system (10,000 watts) should produce about 13,000 kwh in a year. This estimate should be within about 10% of an actual system s production. Actual production will be higher in the summer and lower in the winter, and total output will change if the average solar insolation for the year is different than the long term average. Shading or non-optimal panel orientation will decrease performance as will module age. The panel efficiency is not included in this analysis because it simply determines the amount of panel area that is needed to get the rated output. So, two panels rated at 200 Watts will both produce 200 Watts at peak conditions, but the one with lower panel efficiency will be larger in size. Annual savings can be calculated by multiplying the annual production, in kwh, by the cost of electricity. If the cost to install a system is known, a simple payback period can be calculated. Using a tracking system to orient the panels toward the sun throughout the day would increase total production by about 30%, but is probably not worth the investment considering the cost of tracking equipment and the increased maintenance. It would be simpler, and probably more economical, to spend money on more fixed PV panels instead of a tracking system. A more detailed performance analysis can be done using a free online tool developed by the National Renewable Energy Lab (NREL) called PVWatts 11. This tool accesses meteorological data for the desired location and calculates monthly system production based on entered system parameters. It also determines the local electric rate to calculate monthly savings. There is almost no maintenance associated with a non-tracking solar PV array as there are no moving parts. Also, panels will suffer some performance degradation from dirt or snow accumulating on them, but experience at the WCROC suggests rain is sufficient to keep dirt and dust at bay, and snow typically 12

22 melts away from panels within 4 to 5 days of a storm. However, roof mounted panels on a swine barn at the WCROC were covered with snow for almost the entire month of January in 2016 reducing production to almost zero for the month. Performance could have been improved by brushing snow from the panels, but the reduced sunlight available in January means, at most, about $150 of electricity was lost. This potential income has to be weighed against the time and risk associated with having an employee removing snow from an icy roof. A future trend in solar electric is the utilization of the panels themselves as structure to protect from elements such as sun, rain, snow, and wind. Consider options to realize multiple benefits from the system. This type of setup may add extra costs for example if used for cattle shade (higher, more robust mountings), but could also be used for an outdoor picnic area or keeping vehicles sheltered. Wind Wind turbines rated at less than 100 kilowatts (kw) in size are generally considered small-scale. Smallscale wind turbines are suitable for siting at an individual residence or farm site and can offset some or all of the electricity used at the site. Unlike solar resources, wind is very site specific. The amount of energy that can be harvested from the wind depends heavily on the local geography and weather patterns, nearby obstructions like buildings and trees, and the tower height of the wind turbine. The typical small-scale wind turbine is mounted on a tower that ideally puts the hub about 100 ft. (30m) off the ground. The average speed of the wind in Minnesota at a 30 meter height has been modeled and mapped by the National Renewable Energy Lab (NREL) and is shown below. 13

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24 One of the first steps in developing a wind energy project is to assess the area's wind resources and estimate the available energy. Correct estimation of the energy available in the wind can make or break the economics of a project. The average annual wind speed can be used to make general estimates of a turbine s annual energy production, but direct monitoring by a wind resource measurement system at a proposed site will provide the most accurate data. If direct monitoring is done, it should be done for at least one full year. Direct monitoring of the wind resource may be cost prohibitive for a small scale system. A good overall guide on this subject is the Wind Resource Assessment Handbook 12 produced by NREL. The wind power equation shows wind power is most affected by the wind speed: Wind Power = ½ x Air Density x Swept Area x Velocity 3 The wind speed is taken to the third power so doubling the wind speed will result in eight times more power. This is why even small increases in the average wind speed can lead to significantly more power production. The average wind speed increases with height so an incrementally taller tower may be worth the extra investment due to the increased power available. The swept area is determined by the length of the turbine blade and air density is determined by altitude above sea level and air temperature. So, at a given site, the most power is produced with the longest blade and tallest tower. This will be the most expensive option as well, so a careful analysis is needed to determine the most economical solution. Wind turbines may generate DC or AC electricity. In either case, the electricity must be conditioned to be fed onto the local utility grid or used to charge batteries. Wind electric systems can be configured as grid tied or stand-alone systems in the same way as solar PV systems, and wind power inverters should also have the 1741 UL listing. The listing is much less common with wind systems than solar systems and, therefore, would be a good question for potential installers. Since wind speeds increase with height, a tower is used to get the turbine as high as possible. Air turbulence is created by obstructions to the wind like buildings, trees, and hills. Air turbulence decreases the performance and longevity of a wind turbine and is another reason to mount a turbine on a tower as high as possible. A general rule of thumb is to have the bottom of the rotor blades at least 30 feet above any obstacle within 300 feet of the tower. There are two basic types of towers: self-supporting (free standing) and guyed. Most home wind power systems use a guyed tower as they are the least expensive and easiest to install. Guyed towers require more installation area since the guy radius must be one-half to three-quarters of the tower height. Tiltdown towers have a hinge mechanism at their base which allows them to be raised and lowered through a winching action or hydraulics. Although tilt-down towers are more expensive, they offer an easy way to perform maintenance on smaller turbines usually 10 kw or less. Performing maintenance 100 feet in the air is problematic and somewhat dangerous. Another potential cost if the tower is not self-lowering, a crane may be required for installation AND potentially for repair. Tilt-down towers also offer a way to protect the turbine in the event of severe weather. 15