Renewable Energy Generation and Storage For Agricultural Use in the San Luis Valley

Transcription

1 Renewable Energy Generation and Storage For Agricultural Use in the San Luis Valley Feasibility Study Report Primary Researchers: Greg Martin, Masters Candidate Jonah Levine, Masters Candidate Research Support: Dr. Frank Barnes, Distinguished Professor Dr. Ewald Fuchs, Professor University of Colorado at Boulder Feasibility Study: Final Report:

2 - Report Contents - 1 Technical Report Overview Renewable Energy Sources Analysis Photovoltaic Solar Panel Array Wind Turbine Generation Irrigation Loads and Operation Crop Requirements Irrigation Power and Energy Requirement Assumptions Economic Incentives Energy Storage System Technical, Cost and Benefit Analysis Selected Energy Storage System Options Analysis Matched DC Motor Pump System With Battery Storage (Option 1) System Description and Operation Cost and Benefit Analysis Matched DC Motor Pump System With Grid Storage (Option 2) System Description and Operation Cost and Benefit Analysis AC System With Grid Storage (Option 3 - Reference) System Description and Operation Cost and Benefit Analysis AC System With Surface Level Water Storage (Option 5) System Description and Operation Cost and Benefit Analysis AC System With Compressed Air Storage (Option 6) System Description and Operation Cost and Benefit Analysis Discarded Options Pumped Water Energy Storage In Elevated Water Tank (Option 4) Technical Analysis/Discussion Reasons For Option Rejection AC System With DC Battery Storage (Option 7) Technical Analysis/Discussion Reasons For Option Rejection Energy Storage Option Comparisons Technical and Operation Pros and Cons Cost Comparisons Summary and Conclusions

3 1 Technical Report Overview This report compiles data and analysis of several renewable energy generation and storage systems for agricultural use in the San Luis Valley of southern Colorado. The researchers have gathered information on two real world irrigation systems to determine many of the required system characteristics, sizing, and operational requirements. An initial feasibility study laid the groundwork for this report by identifying seven system design options, and selecting five of them for focused analysis and comparison in this report. The energy storage systems under study were designed and selected assuming that the power source is a large photovoltaic solar panel array. In addition to solar, this report analyzes the viability of using wind turbine generators as a power source. 2 Renewable Energy Sources Analysis 2.1 Photovoltaic Solar Panel Array The sun is powerful in the San Luis Valley. The region receives the most solar radiation in the state of Colorado, as seen in Figure 2. The amount of incident sun energy in the SLV is significant, and comparable to that in Phoenix, Arizona. Figure 1 shows a Colorado state map of direct normal solar radiation, Figure 2 plots the average daily insolation per month in Alamosa, Colorado. The availability of land area and high solar radiation in the San Luis Valley make it a promising location for solar power generation. Energy storage solutions for use with a photovoltaic solar array were considered. San Luis Valley Figure 1. Average Direct Normal Solar Radiation (Insolation) in Colorado 3

4 Insolation In San Luis Valley SOURCE: NREL Website Solar Data tables. PVWatts Estimation Program Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month 1980 Data Fixed Axis PVWatts Two Axis PVWatts Single Axis PVWatts Figure 2. Insolation incident on the ground and on solar panels with fixed, single and dual axis control. SOURCE: NREL Website Solar Data tables. PVWatts Estimation Program An initial feasibility study was conducted for energy storage options operating with a photovoltaic solar panel array. The sizing and characteristics of such a solar array were analyzed. Cost estimates, including rebates and incentives, and approximate pay back period were evaluated and are summarized in Table 1. For most energy storage options, the renewable energy source (solar array in this case) represents the major cost of the overall system. Table 1. Photovoltaic Solar Array Cost Estimation. Total Output Power [kw DC] 57 Panel Array Capital Cost [$] 339, Panel Mount Cost [$] 46, Estimated $7/watt [$] 399,000 $2.00/Watt Rebate [$] -114,000 30% Tax [$] -85,500 Initial Capital Cost [$] 199,500 Annual Electric Bill [$] 9,000 Pay Back Period [years] 22 Two study sites have been used to collect consumption data giving the study a well-rounded look at the generation and storage challenge. Both sites are in the SLV. The first ( Solar A ) site uses San Luis Valley Rural Electric Cooperative (SLVREC) and is privately owned and operated. The other site ( Solar B ) uses Xcel Energy and is run by CSU Agricultural Extension service. Both electric utility providers have different rates of charge and reimbursement. Cost and technical data for the Solar sites are shown below in Table 2. Table 2. Solar Sources A and B Capital Cost and Technical Specifications SOLAR A SOLAR B Panel Part Make/Model Sharp / NE-170U1 Sharp / NE-170U1 Panel Height [m] Panel Length [m] Panel Area [m2] Panel Efficiency

5 Panel Max Rating [kw] Panel Avg Output 80% Insolation] Panel Per Unit Cost [$] Number of Panels Required Array Cost [$] 339, , Face Area of Panels [sq. m] Panel Mount Cost [$] 46, , Panel Voltage [V] nom (max) 24 (34.8) 24 (34.8) Series Panels Parallel Panels Total Output Power [kw DC] Table 3 gives cost estimates for the two solar array sizes. Annual electric bill information has been taken from the two case study sites. Solar panel cost data was found on the internet. Table 3. Cost estimate of solar arrays. Solar A Solar B Total Output Power [kw DC] Solar Array Capital Cost [$] 339, , Panel Mount Capital Cost [$] 46, , Aprox $7/watt [$] 399, ,000 $2.00/Watt rebate [$] -114,000-94,000 30% tax [$] -85,500-70,500 Initial Capital Cost[$] 199, ,500 Annual Electric Bill [$] $9,000 $7,500 Pay Back Period [years] Wind Turbine Generation (Future work) 3 Irrigation Loads and Operation 3.1 Crop Requirements To correctly size the solar array and energy storage systems, crop irrigation requirements were studied. While there is no set irrigation schedule, the seasonal water needs can be assessed and averaged per day, per week and per month. A potato crop was assumed, requiring approximately 12 inches of water (in addition to sparce rainfall) per season. The total water need was increased to account for evaporation and runoff. The growing season is assumed to be five months long, April through August. Table 3 summarizes the potato crop irrigation requirement and average irrigation time per day, per month and per season. A standardized irrigation water flow of 850 gallons per minute was assumed. Assuming a crop area of 130 acres takes 6 hours to impart 0.1 inches of water, the average irrigation time for each segment is also estimated in Table 3. Table 3. Seasonal irrigation amount and time assumptions. Seasonal Water Needed Monthly Water Needed Weekly Water Needed Daily Water Needed Irrigation Time Per Season Irrigation Time Per Month 16 inches 3.2 inches 0.8 inches 0.11 inches 960 hours 192 hours 5

6 Irrigation Time Per Week Irrigation Time Per Day 48 hours 6 hours 3.2 Irrigation Power and Energy Requirement Assumptions The major power consumer for irrigation is a 50 to 75 horsepower well/pressurization pump that draws water from the aquifer (depth 150 feet) and provides pressurized water to the irrigation head. Several motor wheel drives move the irrigation arm in a circle. Chemigation and fertigation pumps feed chemicals and fertilizer into the irrigation water. The connected load ratings assumed and instantaneous load requirement are summarized in Table 4. Table 4. Assumed irrigation electrical load ratings. Well/Pressurization Pump Wheel Drives (x7) Chemigation/Fertigation Maximum Instantaneous Load Residential Load 45 kw 5 kw 10 kw 50 kw 7.5 kw The main well/pressurization pump provides 850 gpm of water at 50 psi. For some low energy irrigation systems (LEPA) the pressure could be as low as 7 to 10 psi, significantly reducing the power required of the main pump. This study assumes that 3 days worth of enough energy storage to continue to meet the average daily irrigation need is required of any proposed power system. 4 Economic Incentives Rebates Defining the rebates for Amendment 37 (A37) renewable energy systems greater then 10kW is challenging. The rules from zero to ten kw are published by Xcel Energy and agreed upon by the Public Utilities Commission (PUC). Xcel Energy will rebate customers $2 per watt of solar panels installed on customer premises, up to 10,000 watts (or 10 kilowatts). Also as part of the program, the company will purchase Renewable Energy Credits (RECs) generated by customer systems for $2.50 per watt. These credits then will be counted toward the company s RES requirements under Amendment 37. The combination of the rebates and the credits will generate a total return to customers of $4.50 per watt 1. Rebates for systems above 10 kw are still unpublished but some reasonable assumptions can be made. A37 specifies that utilities be required to offer customers a rebate of $2.00 per watt and other incentives for solar electric generation 2. Systems above 10 kw in size and below 100 kw can count on a minimum rebate of $2.00/watt installed. Looking at the published 10 kw example of $4.50/watt installed we can see that the rebate is in excess of the $2.00/watt requirement. The question whether or not systems larger then 10 kw receive the same favorable treatment. For a conservative pricing model, we will use the $2.00/watt figure and note that a more favorable incentive is a possibility. 1 Xcel Energy News Release, viewable online at (April 25, 2006) 2 Colorado s 37 th Amendment, A37, viewable online at (April 25, 2006) 6

7 Solar System Federal Incentives Federal incentives are available for home owners and businesses for systems placed in service in 2006 and This assistance in the form of a 30% corporate tax credit equipment installed from January 1, 2006 through 2007, the credit is set at 30% of expenditures for solar technologies. 4 Helpful financing assistance from the federal government was available through this year 2006 from section 9006 of the Farm Bill, this may be available into the future but it was noted for potential cut by the current Presidential Administration. Potential changes to federal incentives will be important to track. The 2002 Farm Bill started to tap the potential of rural America to provide clean renewable energy. Several programs in the 2002 Farm Bill helped to jump start growth in biofuels, biogas, wind power, solar and energy efficiency The successful Farm Bill - Clean Energy programs are a win-win-win for farmers, rural economic development and the environment. They produce a new income stream for farmers and ranchers, create jobs and enhance rural economic development, and provide environmental quality benefits for everyone. The cornerstone of the Energy Title of the Farm Bill is Section 9006, the Renewable Energy and Energy Efficiency Investments program. 5 The 2002 Farm Bill provided creative financing options for rural communities, specifically farmers and ranchers working toward using clean energy. This bill (section 9006) includes grant opportunities up to 25% of $200, on a competitive basis. 5 Energy Storage System Technical, Cost and Benefit Analysis Four major techniques for storing energy on a small scale were considered: Pumped water storage, compressed air storage, battery bank storage, and grid storage. Grid storage refers to the practice of generating energy back into the grid power system to gain energy credits so that this energy can be reclaimed at a later date, though for no longer than a year, and at no additional cost. Two categories of systems emerge as candidates for use with solar power. One category of designs utilizes a DC driven motor pump, powered directly from the solar array. The benefit of this method is that efficiency losses in power conditioning are eliminated. Furthermore, a DC motor and pump can be effectively matched to the solar characteristic to enable maximum power deliver over a wider range of solar array operation points. The drawback to this method is that existing pumps, and most other loads require AC power. The second category is the common standard AC system. DC power from the solar array is conditioned by power electronics, introducing efficiency losses and system cost. There is less opportunity to match the motor pump load to the solar array characteristic with this option. 5.1 Selected Energy Storage System Options Analysis Matched DC Motor Pump System With Battery Storage (Option 1) System Description and Operation Cost and Benefit Analysis 3 Federal tax incentives viewable online (April 25, 2006) 4 Federal tax incentives viewable online (April 25, 2006) 5 Energy Title programs of the Federal Farm Bill and energy efficiency and renewable energy opportunities (April 26, 2006) Environmental Law and Policy Center Farm Bill Clean Energy( 7

8 5.1.2 Matched DC Motor Pump System With Grid Storage (Option 2) System Description and Operation Cost and Benefit Analysis AC System With Grid Storage (Option 3 - Reference) System Description and Operation This system has been implemented effectively in several cases in the US. No on-site energy storage mechanism is used. Rather, grid storage is utilized to accumulate energy credits with the power utility provider, and these credits are traded for grid power when the solar source is not available. It is expected that this system represents the lowest cost solution. However, it is unclear how different utility power providers will handle use of the grid in this fashion. If many agriculturalists adopted this scheme, it may present a larger scale energy availability and storage problem for the utility provider. A schematic of this system is included in Appendix I Cost and Benefit Analysis AC System With Surface Level Water Storage (Option 5) System Description and Operation Pumped water (or hydro) energy storage has proven to be a simple and cost effective energy storage method, especially given suitable geographical characteristics and water availability. Using pumped water to provide irrigation water pressure and water supply simultaneously seems a logical solution for on-site energy storage in agricultural settings. The researchers chose to analyze a storage solution that stores water in a reservoir at ground level. Since significant irrigation pumping power is used to draw well water from a 150 foot deep aquifer up to ground level, this energy could be stored as intermediate energy storage in a reservoir at ground level. Water would be pumped from the aquifer to the surface reservoir using energy from the solar array. Then, the additional energy needed to provide pressure for the center pivot irrigator is supplied by the solar array or grid power. Energy cost savings are realized because it takes less power to irrigate because water does not need to be drawn from the aquifer. Of the total 60 hp (45 kw) used by the conventional main well/pressurization pump, 30 hp is for drawing from the aquifer and 30 hp for irrigation pressure. While the existing AC well pump can be used, an additional boost pump is added to the system. A schematic of this pumped water storage system is included in Appendix I Cost and Benefit Analysis AC System With Compressed Air Storage (Option 6) System Description and Operation For the system size in question (57kW), storage via Compressed Air Storage (CAS) as potential energy has not been a traditional choice, but the initial feasibility investigation has found it to be potentially viable. Design characteristics that represent a technically feasible system have been studied. Challenges in implementing CAS have been identified. The CAS option is a complex system requiring several precision components with moving parts, a number of electrical power inputs, and heat energy input. Significant attention must be paid to thermal management in order to optimize the system and ensure safe operation. A preliminary model has been developed to estimate critical system design characteristics given required operation constraints. This model largely focuses on the thermodynamics of the compressed air management, and ties the thermodynamic performance to power and energy inputs and outputs. 8

9 To illustrate the important characteristics of the CAS system, this summary begins with a discussion of an over-simplified CAS solution model. This model will facilitate design optimization of more efficient and effective features. Benefits and drawbacks to our initial model are identified which allows us to propose a rudimentary though technically feasible system design. An overall schematic of the CAS system is included in Appendix I. The over-simplified CAS system driven by the solar array has the following characteristics, benefits and drawbacks:! A single stage commercial air compressor, driven by an electric motor, pressurizes an air storage container to 1000 psi in a volume of about 512 cubic meters (this is a cubic storage container with sides of 8 meters). The motor is powered ( charging ) from the solar array source when surplus power is available.! The temperature of the air in the container, once compressed, with no active cooling applied, would reach about 900 F. This temperature is too high to be compatible with most practical storage container designs. The researchers propose a reinforced concrete bunker to be used for the storage container. Maximum temperatures much less than 900 F are desirable to be compatible with this type of container. In addition, a storage container at 900 F could be considered a safety hazard. Costs of such a tank, which could store on the order of 512 cubic meters of air at 900 F and at 1000 psi, are not available to the researchers at this time.! Beveled or tapered input and output pressure nozzles would be required to reduce turbulence and would contribute to additional efficiency losses.! Once the super-heated air in the container cools back down (after a few days), much of the input energy used to compress the air has been lost in the form of dissipated heat, and the pressure in the container has been reduced to less than the original pressure of 1000 psi.! Pressurized, ambient temperature air is released into an air driven turbine, which powers an electric generator via a common drive shaft. The generator output electricity must then be rectified and inverted before it is useful to the electrical loads.! The air expelled from the container to the air turbine during energy generation would at ambient temperature, and unless it is heated, the air turbine will not generate efficiently.! While electric motor drive/generator efficiencies are better than 90%, compressor and turbine efficiencies are generally in the range of 10-40%, (40% is assumed for this analysis) greatly increasing the amount of input electrical energy required to store and then extract the necessary amount of energy from the tank. 9

10 Table 5. Compressed air system estimation model for charging of the container. Pneumatic Storage (Compressor) Ambient Pressure kpa 1.00 atm psi Ambient Temperature K C F Container Pressure Rating kpa atm psi Container Length 8.00 m ft Container Width 8.00 m ft Container Depth 8.00 m ft Container Volume m l gal gamma for air (polytropic) 1.30 inlet air density 1.29 kg/m3 moles of air in tank at STP mol mass of air in tank at STP kg Compressed Air Temperature K C F moles of air in pressurized tank mol air density at 500psi, 664K kg/m3 Air Mass kg N lb Maximum Air Mass Flow Rate 0.05 kg/s Compressor Efficiency 0.40 Compressor Power Motor Efficiency 0.90 Motor Power Required For Storing kw hp Charging Time h Energy Applied To Motor kwh Energy Stored In Container kwh Table 6. CAS model for turbine generation from energy stored in the container. Pneumatic Generation (Turbine) Air Turbine Input Temperature K C F Air Turbine Input Pressure kpa atm psi Total Energy Stored In Tank kwh Air Turbine Efficiency 0.40 Air Turbine Output Power kw Air Turbine Output Energy Total kwh Generating Efficiency 0.93 Generator Power Output kw Generating Time h Generated Energy kwh The major additions needed to optimize this system include cooling of the stored air, replacing the energy lost when the container cools, and the subsequent re-heating of the air as it is fed to the air turbine. In large scale, conventional compressed air energy storage (CAES) systems, these problems are addressed by adding an air cooler at the compressor output, and an air heater, or expander, at the turbine inlet. Normally, a conventional gas turbine is used to generate power from the stored air. These turbines combine a compressor stage, an expander powered by natural gas, and a turbine stage. Removing the heat from the compressor output requires significant additional energy input. As a first order approximation, we assumed the amount of energy input to the air cooler is equal to the amount of energy lost when the super-heated container cools to ambient temperature. An estimate of this energy value is about 1000 kwh. While the final generated energy output of the system remains the same, an additional 1000kWh input is required from the solar source. This results in a longer charging time, increasing it from 88 hours to 108 hours. The model used to estimate the CAS system characteristics is a useful tool, which can be effectively used to parameterize several important variables of the system design. With additional development, this model will 10

11 be instrumental in yielding an optimized system design. To allow for efficient air turbine operation, a heat source implementing an expander stage is proposed. The most effective conventional source of this heat is natural gas. Electricity from the solar array cannot be used during generation because, by definition, solar array power is not available. Grid supplied electricity powering the expander stage is not cost effective compared to the use of natural gas. There are challenges to designing and implementing an effective CAS system on the scale in question. There seems to be opportunity for innovation to improve the efficiency and feasibility of such a storage system. Ideas to mitigate the challenges discussed above include:! A method to capture the heat extracted from the compressor cooling stage and then apply it to the expansion heating stage would increase the system efficiency.! The air cooler could possibly be eliminated if several charging cycles are to be used to compress air into the container, let it cool, and then compress more air. This would maximize the stored potential energy in the compressed air container. This approach requires additional input compression power and time to be spent in charging the storage container. The container air would reach very high temperatures as discussed previously, creating a potential safety hazard, and complicating the container thermal design.! If the stored energy in the container could be utilized when the compressed air is still hot, the round trip efficiency of the storage system could be maximized. However, it is possible that the stored energy would be idle for longer periods of time (and allowed to cool) before it is used.! Solar thermal energy could be used to hold the storage container at the high temperature reached during the compression. Unfortunately, significant cooling of the container would occur at night when no solar thermal energy is available. Several components of the compressed air storage system studied here have little representation in commercially available hardware, most notably the high pressure large volume air storage container, generating air turbine, high pressure air cooler, and high pressure air expander. Although cost estimates are not available for this study currently, typical of new technology, it is estimated that the CAS system has a high capital cost relative to the other alternatives explored in this study. CAS and CAES systems remain an important area of study as energy storage methods to complement renewable energy sources. Compressed air storage is a method that does not depend on geography and could be utilized virtually anywhere energy storage is required. Further research and the development of optimized, more economical system designs that are scalable to various system sizes are required for detailed cost-benefit analysis of CAS and CAES systems Cost and Benefit Analysis 5.2 Discarded Options Pumped Water Energy Storage In Elevated Water Tank (Option 4) Technical Analysis/Discussion A logical solution to this energy storage challenge is to store the energy in the pumped medium, water. Further, if the water could be given sufficient potential energy so that irrigation could take place with no additional energy input, the system would have added flexibility and reliability. To accomplish this, water would need to be pumped into a large tank at sufficient elevation to provide the required head pressure of 50 psi at the irrigator. Table 7 summarizes the key design points for this energy storage solution. Table 7. Pumped Water Storage At Elevation Design Information hours of application from storage tank 0.33 inches of water for crop irrigation acre crop gallons per minute application rate cubic meters of water in storage tank gallons of water in storage tank 11

12 10.00 meters tank radius meters tank diameter meters tank height pounds of water stored tons of water stored 115 foot elevation of storage tank Reasons For Option Rejection Examination of the design characteristics in Table 7 illustrates that some 4800 tons of water would need to be stored in a 115 foot high storage tank. This design is assumed to be impractical. The only way to make this scale of water storage possible is to utilize a hill or high land formation near the crop and carve out a reservoir. It is not assumed that such a land formation is available in all cases AC System With DC Battery Storage (Option 7) Technical Analysis/Discussion An energy storage system consisting of lead acid batteries connected to the solar output DC link has the characteristics summarized in Table 8. Table 8. Battery Bank Specification for Energy Storage. Battery Storage System Specifications Required Power Output kw Required Time h Required Energy Output kwh Battery Capacity Over Time Requirement 2.78 A Battery Bank Capacity Over Time Requirement A Number Of Batteries In Series Batteries Number Of Batteries In Parallel Batteries Battery Bank Voltage Vdc Battery Bank Power kw Battery Bank Energy kwh Battery Bank Volume cu. M Batery Bank Length m Battery Bank Width m Battery Bank Height m Battery Bank Weight kg Battery Bank Cost $ 24 Year Battery Bank Cost $ Battery Specifications Model Number 8G22NF Battery Make MK / Deka Battery Type Gel Lead Acid Length 0.24 m Width 0.14 m Height 0.24 m Volume 0.01 cu. M Weight kg Capacity Ah Voltage V Cost $ 12

13 It is assumed in the battery bank design that the normal battery can discharge half of its capacity, causing the battery bank size to double Reasons For Option Rejection The AC system battery bank storage option requires 80 gel lead acid batteries. This battery bank would take up about 1,056,000 gallons of space, or 142,000 cubic feet. It would weigh 1.5 tons and costs $12000 for the batteries alone, neglecting containment facilities. Each battery has a 1 year warranty, but can be expected to last about four years. All batteries would need to be replaced 6 times during the lifetime of the system (solar array), giving a life cycle cost of $72,000. While we reject lead acid batteries other battery options may exist including Sodium-Sulfur batteries. 6 Energy Storage Option Comparisons 6.1 Technical and Operation Pros and Cons 6.2 Cost Comparisons 7 Summary and Conclusions 13

14 Appendix I: System Block Diagrams Schematic 1. Standard Solar Array With Grid Storage. Schematic 2. Solar Array With Pumped Water Storage. 14

15 Schematic 3. Solar Array With Compressed Air Storage. 15