Hybrid Power Systems

While wind, PV, and micro-hydro have been commercial technologies for a number of years, their hybridization with fossil fuel generator sets for rural applications are an emerging (not developing) technology.    Such hybrid systems combining renewable energy and fossil energy components have their roots in telecommunications applications in remote sites.  However, the extremely high-value electricity for telecommunication applications results in expensive, extreme reliability designs that are inappropriate for rural electricity service. Furthermore, the village hybrid market is at a very early stage of development, and the design, manufacturing, integration, implementation, and distribution segments of the industry are sparse and immature.

 

Technical Summary

Resource Assessment

Cost, Performance and Project Risks

Technical Summary

There are a large variety of possible hybrid power systems, but the most common types include:

• PV-Diesel hybrids
• Wind-PV-Diesel hybrids
• Wind-Diesel hybrids
• Retrofit systems

These systems can be developed a retrofits of existing diesel mini-grid systems or as new integrated designs.  NREL has developed a series of pilot village power systems that have exemplified these system types.  Other developers that are active in this area include Trama TecnoAmbiental, The Fraunhofer Institute for Solar Energy Systems, and ISET (Institut für Solare Energieversorgungstechnik).

In general, well designed hybrid systems will substantially reduce diesel fuel consumption while increasing system reliability.  In addition to the diesel generator and the renewable energy generator, hybrid systems consist of a battery bank for energy storage, a control system and particular system architecture that allows optimal use of all components.

The diesel fuel savings of hybrid systems are associated with a control strategy and system architecture that allows shutting down the diesel generator when the renewable energy system output is sufficient to carry the load, and uses short-term battery storage to reduce diesel generator start-ups.    In northern climates, an added feature is using both waste diesel heat and surplus renewable energy to supply space heating to community buildings. 

Experience from several of the NREL pilot systems indicates that in many cases hybrid system:

• Provided power at lower costs than grid extension
• Were more reliable than grid power,
• Provided much higher service levels than diesel-only systems
• Were very modular and could be assembled from standardized packages
• Were relatively easy to fully automate

Existing diesel mini-grids can also be retrofit.   Diesel generators currently have the largest share of the existing mini-grid market.   Yet, in many developing countries, ongoing fuel subsidies for these remote diesel systems place a heavy burden on public budgets.   It is possible retrofit existing diesel mini-grid systems with renewable energy generation and battery storage.  This should be an attractive investment for the government agency providing the fuel subsidy in situations where the savings in annual subsidy payments provides a reasonable payback on the investment in the renewable energy equipment.

Resource Assessment

Information on wind and solar energy resources must generally be obtained through locally sources.  General solar resource data exists for hundreds of specific locations around the world, which can be interpolated to a particular project location.  However, wind and other renewable resource data are very site specific, and several years worth of data are needed to ensure that long-term financial calculations are based on the average resource characteristics.

Experience has shown that most national wind maps radically under-estimate available wind energy resources. Very often, wind sensors are sheltered below trees or buildings or actually roof-mounted so that they do not properly deflect the available wind.    In other instances, the equipment/measurements are faulty, such as wind sensors having worn bearings or not being properly calibrated, which leads to under-measurement of the wind resource. 

EBRD and US-DOE are developing new international wind mapping resources.  Private companies now active in new wind resource mapping, and new low-cost wind loggers, specifically designed for small wind applications, are now available.

Cost, Performance and Project Risks

Hybrid systems are potentially very cost-effective solutions to rural AC electricity needs.   For low load applications (< 10 kWh/day), Wind/PV Hybrid Systems are very attractive.   For Larger Applications, Wind/Diesel Hybrids are very attractive as long as a reasonable wind resource is available.  Bilateral and multilateral finance and market stimulation programs should be based on best service at least cost. 

The cost of hybrid systems is currently high compared to conventional diesel mini-grid systems.  However, as is typical of emerging technologies/markets, systems design and industry structure will continue to evolve in concert with the growth in demand and technology development funding, and costs can be expected to decrease dramatically.

Wind and solar energy components and Diesels have complimentary technology characteristics, as shown in the table below.   Together, they provide a more reliable and cost-effective power system than is possible with either wind, solar or diesel alone.