Conventional methods of providing power require expensive pole-drop utility connections, high replacement cost of battery-backup systems, other regular maintenance, or inordinate size and substandard reliability. The proposed device uses an optimized combination of battery and solar power, which when coupled with sophisticated power electronics circuits, is capable of supplying 50 W-hr/day of electricity, continuously, unattended for at least four years. This will improve substantially on conventional, non-electronic, high-maintenance solar-battery systems. It will also be far less expensive and more energetic that devices based on double-layer capacitors. OBJECTIVES: The overall feasibility of the proposed research will be supported by several technical objectives. Feasibility must be demonstrated across four separate objectives: meeting daily load (capability), meeting that load every day regardless of weather (reliability), meeting that load every day for a long time (longevity), and showing that other load profiles in other locations can be handled (adaptability). Technical Objective 1: Demonstrate 50 W-h/day of energy use (daily load capability), which will be shown by direct measurements on a demo unit with artificially imposed worst-case conditions. Technical Objective 2: Demonstrate the capability to meet load demand under a defined weather scenario (reliability requirement). For this project, we have chosen to use a two-day period with minimal ambient light as a reliability requirement. Other requirements may be appropriate if less extreme reliability is needed. The point is to first define a reliability requirement and then show that the device is capable of meeting it. Technical Objective 3: Demonstration of four to twenty years of operation by extrapolation (longevity requirement). This objective will be achieved by using appropriate battery chemistries (not necessarily lead-acid for the harshest environments) and sophisticated, low-power, charge management techniques. We believe that a minimum of four years of longevity in the harshest rural environments, like the Southwest and Texas, is required to be interesting to end users. Such a device could provide up to twenty years of life in mild environments such as the Pacific Northwest, Alaska, and Northeast rural areas. This objective cannot be tested in real-time during an eight-month Phase I, however, we will use historical data, manufacturer life-cycle data, and computer modeling approaches instead. Technical Objective 4: Determine cost and performance trade-offs with scaling and alternate locations (adaptability requirement). One way to solve the scale-up (or down) in energy requirements is to use a stackable, modular device. This objective involves creating the device such that it is modular and hot-pluggable for easy increase in energy output. We will demonstrate this point of the objective with hardware demo units connected in parallel that are proven with measurements to share equally. APPROACH: The technical objectives will be met by performing the following list of tasks. Task 1: Collect information on batteries and solar cells. Finding the best combination is the most significant factor in minimizing the cost and maximizing the performance of the unit. Task 2: Analyze weather data for energy storage and power requirements. This Task will involve collecting the required historical weather data across a number of rural locations. This data will be organized into a database which can be referenced by a computer model of the solar-battery system. For a given combination of solar cells and battery combinations, we can rapidly predict the W-hr/day energy output across many locations over many years. From those situations, we can choose the combinations that are likely to have to lowest cost yet still meet the Technical Objectives. Task 3: Design single-unit energy management circuits. In this task we will carry out the design of the energy management circuits required to achieve the performance of the solar-battery combinations suggested in Tasks 1 and 2. These circuits will be implemented using sophisticated power electronics and controls. Task 4: Design multi-unit control strategy. Once a single unit system has been proven viable, it will be necessary to show how multiple units can be cascaded. This will be more difficult than simply connecting the outputs. A method based on sensorless current mode control (SCM) was developed by SmartSpark for ultracapacitor-based backup power systems. It may be possible to adapt this technique to a battery-based system. Task 5: Fabricate and validate demonstration unit. To carry out this task, first we will use the detailed computer models from Task 4 to assemble the subcomponents in simulation. This will let us test the system quickly and nondestructively for basic concerns like stability and dynamic performance. If necessary, we will make design revisions to correct stability or other dynamic performance problems. Task 6: Fabricate and test multiple, paralleled demo units. This task involves slight modification of the single unit version discussed in previous tasks. Using the method developed in Task 4, the demo unit will be modified and replicated for multi-unit testing. The exact same tests will be repeated as in Task 5, except scaled appropriately to account for the number of units. The outcome of this task will be affirmation of all four Technical Objectives. Task 7: Project reporting and management. This Task involves the technical support ensuring that all tasks are proceeding on schedule and that reporting requirements are met. This task is conducted simultaneously, as necessary, with all the other tasks. It will be carried out by organizing and documenting regular meetings of the technical staff as well as the relevant business management personnel. Documentation will include budget analysis. Costs are accounted for by project, and thus monthly budget reports can be generated to confirm that the project is progressing according to plan, or to identify areas of need
