Dennis L. Larson

Parametric model of solar cooker performance

This paper presents a model for prediction of the cooking power of a solar cooker based on three controlled parameters (solar intercept area, overall heat loss coefficient, and absorber plate thermal conductivity) and three uncontrolled variables (insolation, temperature difference, and load distribution). The model basis is a fundamental energy balance equation. Coefficients for each term in the model were determined by regression analysis of experimental data. The model was validated for commercially available solar cookers of both the box and concentrating types. The valid range of model application includes most of the feasible design space for family-sized solar cookers. The model can be used to estimate the cooking capacity of existing box type and concentrating type solar cookers. It can also be used to find the combinations of intercept area and heat loss coefficient required to cook a given quantity of food in a given climate.

Electro-migration of nitrate in sandy soil

Migration of nitrate to groundwater has become a serious threat in many agricultural areas. This paper presents the results of experimental laboratory tests studying the nitrate gradient developed in response to an electrical potential. Two systems were tested; the first had no flow (closed system) and the second had flow opposite to the direction of the electrical current. A solution of sodium nitrate in sandy soil was used in both systems. The tests showed that the electro-kinetic process effectively concentrated and retained nitrate close to the anode. The movement of NO(3)(-) through the soil column was significantly influenced by the development of a pH gradient. Statistical analysis was performed to determine best-fit equations relating the nitrate gradient to the electrical input and pH gradient. A simple one-dimensional finite difference model was used to predict the pH gradient developed during the electro-kinetic process. The experimental measurements closely agreed with the predicted spatial and temporal distribution of the nitrate gradient for both closed and open system configurations.

Exergy analysis: essential to effective energy management

Energy management which considers quality or capability as well as quantity can better identify appropriate and cost effective use of energy. Energy analysis utilizing the first and second laws of thermodynamics, termed exergy analysis, considers both energy degradation and loss to provide most effective, not just most efficient, management solutions. This article summarizes the basis for exergy analysis and presents examples which demonstrate the benefits of exergy management in agricultural engineering applications.

EFFECTIVE NITRATE CONTROL WITH ELECTROKINETICS IN SANDY SOIL

Nitrate contamination of surface and ground water is a problem worldwide. Nitrate in drinking water presents a human health risk. The major source of nitrate contamination is believed to be nitrogen fertilizer from agricultural fields. Best Management Practices have been developed to guide fertilizer use and minimize nitrogen losses, but they do not address control of nitrate movement from the crop root zone. It is proposed that electrokinetics, an in-situ method, could be used to control nitrate movement, retaining it near the root zone. This article reports results from a study on nitrate movement and pH changes in a vertical, partially saturated sandy soil column subjected to an electrical current. The highest measured nitrate concentration (7155 mg/L) was within 5 mm of the anode after application of a 6 h 80 mA current. The nitrate concentration at the cathode was 1/5 of the inflow solute concentration. The pH was 11 near the cathode, 3.5 near the anode, and showed little change in intermediate layers. Studies of different nitrate concentrations and electrical current levels suggest that 80 mA electrical current applied for 6 h duration might effectively control nitrate migration in similar sandy soil conditions.

Canal Irrigation Allocation Planning Model

A water allocation model was developed to recommend allocation of irrigation water to different crop fields in a canal-based irrigation project. Model components are an irrigation scheduling program to predict irrigation water demands, a crop response model to compute crop yields, and a canal delivery model to check the physical feasibility of water delivery. Multiperiod linear programming is utilized to determine the optimal allocation strategy, which maximizes irrigation benefits. Allocation constraints are irrigation water demand, irrigation water availability, canal delivery capacity, minimum irrigation limitations, and crop response model limitations. The allocation model was validated using crop, soil, canal, and irrigation management data for MAC, a University of Arizona farm.

Nitrate pollution control in different soils by electrokinetic technology

Previous studies found that a small DC electrical current could attract anions to the anode in sandy soil, even with solute flow towards the cathode. Laboratory experiments were conducted in a vertical, partially saturated column with different soils to determine if nitrate transport could similarly be controlled using electrokinetic (EK) technology. Nitrate concentration, pH value, electrical potential difference, and soil water content were measured for three soils at selected times at different distances from the anode. Constant electrical current was applied to the system for 9 h, and measurements continued for a total of 48 h. The results demonstrated that nitrate can be strongly retained near the anode against gravity in sandy soil with an 80 mA (0.5 mA/cm2) current input. When the percentage of clay in the soil was increased, the EK effect on ion movement decreased; the transport of both ions and water were inhibited by fine clay particles. The loamy soil showed a slight increase in nitrate concentration near the anode, but the clayey soil showed no change. An increase in pH near the cathode was seen in all soils. Water content for sandy soil was higher at the bottom of the column and lower at the top of the column, but for loam and clay soils, the lowest water content was found above the cathode near the bottom of the column. Electrical potential difference between the two electrodes showed that the sandy soil required the highest electrical potential difference to obtain the desired current level; loamy and clayey soils required less. For sandy soil, the highest potential difference was found near the top of the column, but for loam and clay soils, the highest electrical potential difference was measured near the bottom, next to the cathode, suggesting that these locations were the critical zones limiting electrical ion transport.

EFFECT OF AN ELECTRICAL INPUT WITH DRIP IRRIGATION ON NITRATE DISTRIBUTION IN SOIL

Soluble chemicals applied to soils have been identified as major sources of surface and groundwater contamination. This research examined the application of a small dc electrical input to attract and retain nitrates in the root zone of drip irrigated barley in small lysimeter trials. Nitrate content near the anode was higher and pH lower, as desired, during a portion of the test. However, the electrical input seemed to have little effect on nitrate distribution at other times, perhaps due to careful management of water and nitrate inputs. These results indicate a potential benefit of an electrical input to the control of nitrate transport in soil.

Electro-osmosis Effectiveness in Reducing Tillage Draft Force and Energy Requirements

Tillage tests were conducted in a laboratory soil bin to quantify the draft force and tillage energy reductions obtained with electro-osmosis. Electrical parameters examined were voltage level and use of one or two anodes; the tillage tool served as the cathode. Tests were conducted in loam and clay loam soils at two moisture contents each at tillage speeds of 3.3 to 7.7 km/h. The application of an electrical potential reduced tillage draft force up to 39% in a loam soil and up to 11% in a clay loam soil in soil bin tests. The greatest draft force reduction in the loam soil was obtained with a 40-V electrical input, one coulter as anode, soil moisture content of 17%, and tillage speed of 6.5 km/h. The largest draft force reduction in the clay loam soil was obtained with a 45-V input at 3.3 km/h. The largest draft force reduction was obtained in the wetter of two test conditions in the loam soil, but in the drier clay loam soil test condition. Draft force decreased linearly with increasing voltage in all soil conditions; one anode yielded greater force reduction than two in nearly all tests. Draft force reduction was similar at all tillage speeds in the loam soil, but draft force reduction decreased with increasing tillage speed in clay loam tests. This research indicates application to a full-size tillage implement and in-field verification are merited to assess commercial potential of electro-osmotic tillage.

Development of a Small Solar Thermal Mechanical Pumping System

The University of Arizona is developing a <1kW sized solar thermal mechanical pumping system that appears to have great potential for meeting small pumping requirements (~4000 liters/day with 10 m lift) in locations with adequate solar resource. This solar pumping system is believed to be applicable to many remote domestic and irrigation applications without access to electricity, relying on diesel power and/or having insufficient wind for pumping and to be a cost competitive, locally manufacturable alternative to photovoltaics. The system consists of solar collector, single piston diaphragm engine, custom valving, condenser and return pump. The piston actuates a lever connected to a lift pump. Testing in Oman is scheduled for Spring, 2003.

Management of an On-Farm Grid-Connected Solar Thermal Electric Power Plant

AN experimental 200 kW grid-connected solar thermal electric power plant, constructed in 1979 on a farm near Coolidge, AZ, was operated for three years to characterize energy performance, determine equipment shortcomings and quantify operating and maintenance requirements. The applicability of such an on-farm solar plant to providing irrigation pumping power was found to depend primarily on operating and maintenance requirements, energy management issues and capital costs. It is concluded that specialized operating and maintenance requirements and high capital costs make on-farm siting of solar power plants unattractive in the United States at present.