D. KARNER, R. NEWNHAM, T. GRAY, J. WISHART, N. FENGLER, AND X. SUN, PHEV GRID INTERACTION EVALUATION. ASSESSMENT OF BATTERY TECHNOLOGIES AND SYSTEM CONFIGURATIONS FOR RAPS DUTY.
Two billion people are still without power to their homes. These people, who are remote from the mains grid, live mainly in the developing nations where abundant renewable energy from the sun and/or wind is available. Since the cost of grid connection can be prohibitive, the concept of the stand-alone, ‘remote-area power-supply system’ (the RAPS system) that incorporates a renewable energy source has been advanced. Following steady technological progress, RAPS systems are now becoming cost-effective in many situations. The design of such facilities, however, is far from optimum and still requires considerable development and refinement. The objective of this study is to evaluate the present status of RAPS battery and system technologies that will be eminently suitable for applications in developing countries. A subsequent target is the fabrication and field testing of those RAPS facilities that are identified as state-of-the-art, with a view to distributing proven technology in areas of need. The following aspects are reviewed: (i) individual system components; (ii) system configurations; (iii) laboratory and field studies of RAPS batteries/systems; (iv) manufacturers/distributors of such facilities; (v) availability and estimated cost of systems in developing countries. Special emphasis has been placed on both the battery and the charge-controller components, as these are considered to be the ‘weak links’ in present technology. Applications for RAPS systems have varied widely. Duties have ranged from wind-powered water pumping, back-up power units for microwave-repeater stations, to domestic electricity supplies. Typically, facilities comprise a battery bank (usually lead/acid), an energy source (solar, wind, micro-hydro or fossil-fuel-based) and a controller. The controller can vary in complexity from a simple mechanical switching device to a sophisticated microprocessor-controlled unit. The design of RAPS systems is determined by several factors. These include: (i) energy demand; (ii) availability of renewable energy; (iii) cost of system maintenance; (iv) site accessibility; (v) available finance. Either alternating current (a.c.) and/or direct current (d.c.) can be produced; the choice depends upon the quantity and quality of the power required. For sites with low energy requirements (i.e., 0.2 to 0.5 kWh/day), a d.c. photovoltaic (PV)/battery hybrid system is normally the preferred option. The same configuration is suitable for loads up to 2 kWh/day, but a small inverter is often included to provide a.c. power. Where more than 2 kWh/day is desired, PV/battery hybrid configurations are disadvantaged due to the high cost of the PV panels. At this level of energy consumption, diesel/battery hybrid systems become viable, provided that the operating costs of the diesel generator are reasonable. For this level of daily energy consumption, wind/battery hybrid systems are also competitive with solar-based designs, given that an acceptable wind regime exists (speed 6 m s-1).The study has involved the formulation of basic design specifications for charge controllers and RAPS batteries. This information will enable customers to acquire acceptable quality components. A detailed list of specifications for batteries has also been compiled to guide the user/consumer who requires ultra-performance from the energy-storage component.
A review of laboratory and field evaluations of different types of lead/acid batteries show that the performances of tubular- and flat-plate versions of both flooded- and gelled-electrolyte batteries, together with flat-plate absorptive glass-microfibre (AGM) designs, have been examined. The results reveal that the two flooded-electrolyte versions and the gelled-electrolyte type (assuming correct charging procedures) deliver acceptable service lives, both in the laboratory and in the field. By contrast, AGM units (at their current level of development) perform poorly in the laboratory and, therefore, are deemed not to be suitable for deep-cycle RAPS service where strict charge control is unavailable. The failure modes of batteries cycled in either the laboratory or the field are similar. Deterioration in performance is related mainly to degradation of the positive plate and, in some cases, to sulfation of the negative plates. Water loss from gelled-electrolyte batteries is negligible if correct charging procedures are adopted.
Manufacturers and suppliers have started to meet the needs of developing countries for RAPS systems. In general, present PV/battery hybrid systems for small houses cost between US$ 550 and US$ 900. These configurations, however, often employ automotive or modified-automotive batteries. It is common for such units to fail prematurely. Replacing this technology with imported, good quality, flooded-electrolyte or gelled-electrolyte RAPS batteries, could increase the system cost by up to $150 and $300, respectively. Nevertheless, the use of such batteries does not guarantee that the minimum accepted service-life of three years will be obtained, given the present level of sophistication and reliability of the control systems.
It is considered that the combination of an improved battery and a purpose-built charge controller, when introduced into small-scale PV/battery hybrid systems, would provide the durability required for the successful operation of such facilities in developing countries.
Analyses shows that calculations of the total energy costs associated with the operation of different RAPS system configurations should be interpreted with caution. Many assumptions and approximations are required for the modeling of RAPS systems, namely: (i) load profile; (ii) efficiency of energy generation device; (iii) inverter efficiency; (iv) battery bank efficiency; (v) fuel cost (diesel-based systems only); (vi) cost and lifetime of system/components; (vii) maintenance costs. Variations in these factors can have a marked affect on the overall energy costs.