Tuesday, February 13, 2007

Hybrid Systems in Developing Countries

A common hybrid system for the application in developing countries generally consists of the following main components:

1. A primary source of energy, i.e. a renewable energy resource;
2. A secondary source of energy for supply in case of shortages, i.e. a diesel genset;
3. A storage system to guarantee a stable output during short times of shortages
4. A charge controller;
5. Installation material (safety boxes, cables, plugs, etc.);
6. The appliances (lighting, TV/radio, etc.).

Usually, a DC/AC inverter needs to be installed additionally. All these components and the problems related to their applications. Hybrid systems are applied in areas where permanent and reliable availability of electricity supply is an important issue. Maintaining high availability with renewable energies alone usually requires big renewable energy generators, which can be avoided with hybrid systems. At favourable weather conditions, the renewable part of the system satisfies the energy demand, using the energy surplus to load the battery. The batteries act as “buffers”, maintaining a stable energy supply during short periods of time, i.e. in cases of low sunlight or low wind. Moreover, the battery serves to meet peak demands, which might not be satisfied by the renewable system alone. A charge controller regulates the state of load of the battery, controlling the battery not to be overloaded. The complementary resource produces the required energy at times of imminent deep discharge of the battery, at the same time loading the battery. Storage systems in hybrid systems in developing countries are usually battery aggregates maintaining a stable output over a time frame of one or more days. Rotating masses can be used for shorter time frames (seconds), combustion aggregates need to be used for medium- or long-term storage. A future option might be the hydrogen fuel cell.

Technologies for Hybrid Systems in Developing Countries

1. PV/Diesel

Combining Photovoltaic arrays and a diesel genset provides a rather simple solution and is feasible for regions with good solar resources. PV/Diesel hybrid systems require a DC/AC-inverter if appliances need alternating current, since PV modules provide direct current. Compared to the common solution for rural off-grid electrification using diesel gensets alone, the hybrid solution using photovoltaic offers great potential in saving fuel. Experiences show annual fuel savings of more than 80% compared to stand-alone mini-grids on diesel genset basis, depending on the regional conditions and the design of the system The CO2 emissions decrease correspondingly. Naturally, the observed fuel saving varies over the year. The solar generator can provide about 100% of the electricity during summertime, while in winter this figure is less. Typically, in climatic regions like Germany a PV/Diesel hybrid system is designed to provide around 50% of the electricity from photovoltaic during winter, the rest being supplied with the diesel genset.

2. Wind/Diesel

Wind/Diesel combinations are, in principal, built up in the same way as are PV/Diesel systems. From a perspective of financial competitiveness, they can be applied in regions where average wind speed is around 3.5 m/s already. If wind speed is sufficient, the wind turbine is in charge of the provision of energy. During short periods of time with low winds, the battery maintains a stable system, being replaced by the diesel generating set when low winds occur over longer periods of time.

3. PV/Wind and PV/Wind/Diesel

In some regions the exploitation of both wind and solar resources can become favourable, i.e. at coastal or mountain areas with high degree of solar radiation. Of utmost importance is here that wind and solar energy supply complement each other so that energy provision is possible over the whole year. While for the other hybrid systems applying diesel gensets, the objective in designing the system is to maximise the exploitation of the renewable energy resource, the situation is different
for PV/Wind systems. Here, accurate assessment of the resources is essential for the decision on the appropriate system design. A PV/Wind hybrid system is able to provide energy all time of the day, if weather conditions are favourable. However, breakdowns in energy supply are possible, which is not suitable for some non-household applications, i.e. hospital electrification. Thus, a PV/Wind hybrid system might ideally be supported by an additional diesel generating set for times of extremely unfavourable weather conditions. The PV/Wind/Diesel hybrid system has proven successful in Germany, being highly reliable and resulting in a further reduction of diesel compared to other hybrid systems. This is obviously due to the fact that PV/Wind/Diesel hybrid systems involve a higher share of renewable energy resources. For the application in developing countries, however, it must be doubted whether this effect of further reduction of diesel use can trade off the higher investment and operation costs.

4. Wind/Large Hydropower

On a seasonal basis, the two resources wind and hydropower tend to complement each other to some extent. Especially in winter, when river flows are low, wind has the potential to take over electricity supply. However, during late summer, both resources might become low, and the combination of both is then disadvantageous. Moreover, while hydro generators on rivers are usually at lower levels, wind resources are better at high elevations. For constant electricity generation, another energy resource would therefore be necessary. Since the combination of wind and hydropower offers just limited advantages, it is unlikely that these resources are combined in a project in developing countries, since this opportunity does not seem economically attractive. However, for some locations the situation might be different, so that the feasibility of Wind/Large Hydropower systems needs to be assessed for each case individually.

5. Wind/Micro-Hydro and PV/Micro-Hydro

While hybrid systems with large-scale hydropower generators seem unattractive, microhydropower is more feasible. Micro-hydroelectric generators are turbines that are able to operate under low elevation head or low volumetric flow rate conditions, being suitable for small rivers. Where rivers have inconsistent flow characteristics (dry in summer, frozen in winter), a hybrid system applying wind or PV support can be attractive. A careful assessment of water resources is therefore essential.

6. PV-Wind-Bio Energy

Hybrid Systems with PV-Wind or PV-Diesel or PV-Wind- Diesel are common now in developing countries for decentralized power solutions. But Renewable Hybrid Systems with Bio Energy mix is not so common, but now gaining importance in countries where there is good possibility of utilizing biomass resources. The installation of hybrid renewable energy supply systems based upon PV, wind and bio-energy becomes an optimal energy solution, especially in rural areas where for various reasons the potential for development of various renewable energy systems exist. Also renewable energies such as solar, wind and biomass are becoming popular in countries which emphasize clean energy development in national energy policy. Photovoltaic technology, wind technology and bio energy can be combined together to form a renewable energy hybrid energy system. Such combinations can make use of the best of each, while offsetting weaknesses. This idea basically attempts to establish an optimal energy system as a whole, synthesizing these renewable energy sources, and simultaneously improving the quality and availability of power.


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