Hybrid technology systems combine two or more technologies with the aim to achieve efficient systems. Possible combinations are: wind-solar photovoltaic (PV) hybrid systems, wind-diesel hybrid systems, fuel cell-gas turbine hybrid systems, wind-fuel cell hybrid systems, etc. (see the short descriptions below). Hybrid systems combine numerous electricity production and storage units to meet the energy demands of a given facility or community (Solar Energy Technologies Program, 2006). They are ideal for remote and isolated applications such as communications stations, military installations, islands and rural villages.
Hybrid technology systems combine two or more technologies with the aim to achieve efficient systems. Possible combinations are: wind-solar photovoltaic (PV ) hybrid systems, wind-diesel hybrid systems, fuel cell-gas turbine hybrid systems, wind-fuel cell hybrid systems, etc. (see the short descriptions below). Hybrid systems combine numerous electricity production and storage units to meet the energy demands of a given facility or community (Solar Energy Technologies Program, 2006). They are ideal for remote and isolated applications such as communications stations, military installations, islands and rural villages.
The example of a solar-based hybrid system in combination with wind energy is shown in the Figure below. In this combination, the wind/engine generator acts a backup supply for the AC (alternating current) loads which can be supplied directly to the load without the use of inverter units; the electricity generated from PV is DC (direct current) by nature.
A PV-wind hybrid system is composed of the core part constituting of PV modules and a wind turbine, a DC-AC inverter, batteries, a charge controller regulator, and a backup power resource for battery storage systems (Dayu, no date). PV modules convert sunlight into direct current electricity and they operate using the semiconductor principles that govern diodes and transistors (Patel, 1999). The PV modules can be wired together to form a PV array, which increases the available voltage and increases the available current. However, the power produced is the same in both combinations. A typical PV module measures about 0.5 m2 and produces about 75 Watts of DC electricity in full sunlight. It costs about € 290 and has a lifetime of over twenty years (Dayu, no date). For detailed descriptions of wind energy technology (Patel, 1999).
Overcharging of a battery by the PV array and wind turbine is prevented through a charge controller regulator. Most modern controllers maintain system voltage regulation electronically by varying the width of DC pulses sent to the batteries through a phenomenon called pulse width modulation (PWM). Backup power resource can be maintained either from a generator or from the utility grid when too much energy is consumed or when there is not enough electricity generated from the wind-PV hybrid system.
The Figure below schematically shows how a wind turbine can be combined with diesel generators in a hybrid configuration. This combination enables the use of a renewable energy source in remote and isolated areas, where the grid structure is weak, insufficient or even not existing, and the cost of energy often constitutes a considerable part of the local economy (Ken Tec Denmark, no date).
By connecting a wind turbine to a diesel generator back-up system, an uninterrupted power supply can be acquired, thus securing 100% supply. The diesel generator will take over production when the power generation from the wind turbines is temporarily insufficient to cover the grid demand. The wind turbines are virtually always connectable to the existing diesel generator sets. The new Wind-Diesel concept allows the size of the wind turbine generators to exceed the size of the diesel generators. The maximum fuel saving is achieved by declutching and stopping the diesel engine when the supply from the wind turbine generator exceeds the grid demand. The Wind-Diesel hybrid technology has the advantage of using standard control systems, implemented with modern diesel generators that control the voltage and frequency, even when the diesel engine is not in operation. If the energy production from the wind turbines is higher than the grid demand, the frequency is controlled by the use of a dump-load, which can utilise the excessive wind energy for a numerous other purposes (Ken Tec Denmark, no date).
A fuel cell uses hydrogen (or hydrogen-rich fuel) and oxygen from air to create electricity by an electrochemical process without combustion (US Climate Change Technology Program, 2005). The absence of a combustion process eliminates the formation of pollutants such as NOx, SOx, hydrocarbons and particulates and significantly improves electrical power generation efficiency. Further efficiency gains can be realised by integration of a turbine with the fuel cell. The Figure below shows the FCT hybrid concept in a simple form to provide some understanding of the synergy offered and the basic relationships of components (National Energy Technology Laboratory, no date).
In this direct operating mode, the fuel cell serves as the combustor for the gas turbine. Residual fuel in the high temperature fuel cell exhaust mixes with the residual oxygen in an exothermic oxidation reaction to further raise the temperature. Both the fuel cell and the gas turbine generate electricity, and the gas turbine provides some balance-of plant functions for the fuel cell, such as supplying air under pressure and preheating the fuel and air in a heat exchanger called a recuperator.
In an indirect mode, the recuperator transfers fuel cell exhaust energy to the compressed air supply, which in turn drives the turbine. The expanded air is supplied to the fuel cell. The indirect mode uncouples the turbine compressor pressure and the fuel cell operating pressure, which increases flexibility in turbine selection. Critical issues are the integration of pressure ratios and mass flows and the dynamic control through start-up, shutdown, emergency, and load-following operating scenarios.
Several successful examples of the implementation of different types of hybrid technologies can be observed throughout the world:
- One of the oldest PV hybrid systems and at the same time the first ‘large scale’ PV system in Europe was installed in 1983 at island of Terschelling in the Netherlands (Lysen, 2000). At the Higher Maritime School ‘Willem Barentsz’ a 43 kWp PV system was coupled to a 75 kW wind turbine and a large battery bank.
- A second example is found on Curaçao, the Netherlands’ Antilles (National Energy Technology Laboratory, no date). Since March 1984 the local radio station ‘Radio Hoyer’ uses a PV powered transmitter, with a battery and a diesel backup. The system is installed on the top of the mountain Tafelberg, and is remotely monitored from the capital Willemstad.
- The Tortoise Head Guest House on French Island, Victoria, Australia, generates its power from a remote power wind and PV hybrid system that has been operating since 1995 with support from UNEP (UNEP, 2003). The Guest House is located 150 m from the seashore, which makes it an ideal site for a wind turbine. The system includes: 10 kW wind turbine; 840 W PV array; 2 diesel generators of 15 kW and 25 kW; battery storage (wired to produce a system voltage of 120 Volts DC); and a 10 kW inverter to convert the DC into the Australian standard of 240 Volts AC and 50 cycles per second. The energy uses of the Guest House include: electricity for lighting, water pumping, cold room, freezer, dish washer, domestic appliances, communication equipment and some heating, LPG for water heating and cooking, wood from fallen trees for space heating; solar water heaters to pre-heat water; and diesel for back up electric generator. The Guest House consists of six large bedrooms (for 2-6 people each), 5 double-bed cabins and meeting/conference facilities. About 68% of the energy comes from wind, 11% from PV and 21% from diesel. The Guest House continues to reduce diesel and LPG  consumption through the use of additional solar water heaters  and energy efficiency measures.
- A wind-PV hybrid system is being used at the Samunsan Forest and Wildlife Sanctuary, 60 KM North of Kuching, in Sarawak, Malaysia (UNEP, 2003). The population of the community fluctuates between 20-70 people, including children who return to the community on weekends, tourists, and scientists. The facilities of the Sanctuary include a dormitory, bungalow, guestrooms, office, amenities block, store rooms, boat shed and power shed. The objectives of installing the system were to: provide reliable ‘grid quality’ power supply 24 hours a day; power refrigerators and freezers for tourist services, health, and preserving scientific specimens; reduce environmental impacts; reduce costs; reduce dependence on fossil fuels; minimise potential supply disruptions; enable the community, tourists, and researchers to work and study in the evenings; and reduce the risk of fire associated with the use of candles or kerosene lamps. The system includes: 2.5 kW wind turbines mounted on a 26 m tower; a 900 W PV array; 2 lead acid batteries storing 2 kWhs; 5 kW inverter; 30 kW diesel generator; and remote monitoring equipment. The community has been trained to perform all maintenance activities, which has also increased the community’s appreciation of the system. The wind turbine generates the largest proportion of electricity over the year while in the summer the PV output is at its maximum. The diesel generator is mostly used in the summer, due to periods of low windspeed and an increase in electricity demand arising from tourism, research and community activities. The system was installed in 1997 at the cost of USD 60,000.
- In 1998, a wind/PV hybrid system was installed in Point Hick lighthouse which was converted to a tourist resort in Southeast Victoria, Australia (UNEP, 2003). The resort consists of several accommodation cottages and a low-cost bunkhouse for low budget tourists. The resort is situated in the Cann River national park. The objectives of this hybrid system were: to meet all the electricity demands of the managers and tourist cottages; to reduce the use of diesel operation; to reduce the costs of diesel fuel; and to reduce the environmental impacts from using fossil fuels. The systems consisted of a 10-kW wind turbine on an 18 m tower with 550 W PV array and a 20-kW diesel generator. The inverter used had a capacity of 10 kW. The storage system consisted of a 120-kWh lead acid battery storage. The wind turbine provided an average of 42 kWh/day at a wind speed of 5-6 m/s while the PV array generated a daily average of 2.8 kWh under 5 hours of direct sun. The total system cost amounted to USD 65,000.
- Holwell Farm within the Dartmoor National Park, in Devon, UK, is using a 20-kW Remote Area Power Supply (RAPS) system incorporating a wind turbine system, 20-kWh battery storage and a backup 25-kW diesel generator (UNEP, 2003). The system provides electricity for agricultural activities, bed and breakfast tourist accommodation and other domestic uses. The farm is located 2.5 km from the nearest electricity grid. The three-blade wind turbine has a rotor diameter of 8.8 m, a hub height of 24.4 m and is mounted on a lattice tower. An automated control system ensures that AC power is always available and switches to the diesel generator when batteries are 80% discharged or when electrical demands are high.
- Costa de Cocos is a small scuba diving and fishing resort in Southern Quintana Roo, Mexico, with 12 houses, a restaurant/bar, dive shop, and a workshop. The resort was previously powered by a succession of small (5-20 kW) diesel generators operating just four hours each evening. However, in 1996, a RAPS system consisting of a 7.5-kW wind turbine, battery storage, and two 5.5-kW inverters were installed to provide the resort with electricity throughout the day (UNEP, 2003). The wind turbine is placed on a 24-m tower with protection against salt corrosion. The batteries are located in a specially designed integrated rack assembly. The system cost is approximately USD 35,000 and has a payback period of 8-10 years.
- Another successful example of the hybrid project installation is the Mexican Hybrid Solar Thermal Power Project  (UNEP, 2003). A solar thermal/natural gas-fired hybrid power plant in Baja California Norte with a total net installed capacity of about 300 MW, including about 30 MW for the solar component has been constructed through this project. The plant is a part of the Comisión Federal de Electicidad system expansion plan.
- The largest European PV wind hybrid system is located on the Pellworm Island in Germany (see http://www.pvresources.com/en/hybrid.php). The PV array has the capacity of 600 kW and will be enlarged with an additional 300 kW array. The first 300 kW array was build in 1983 and the second part was connected in 1992. This hybrid system is grid-connected. The eventual 900-kW capacity will enable the production of nearly 800 MWh/year.
- Another successful example of hybrid technology is a PV-wind-diesel hybrid system in Kythnos Island of Greece (see http://www.pvresources.com/en/hybrid.php ). It has been in operation since 1983. This plant utilises a 100-kW PV array, a 100-kW wind turbine, and a 600-kWh battery. The entire system is connected to the existing distribution grid, which is fed by a 200-kVA diesel generator. Three 50-kVA inverters operate simultaneously to deliver power to the grid. The plant is monitored in order to optimise the amount of renewable energy available to the grid.
- The Wilpena Pound power station  of South Australia combines a 100-kWp PV system, a battery storage of 400 kWh, an inverter and a 440-kWp diesel generator (see http://www.pvresources.com/en/hybrid.php). At night, a computerised smart controller automatically switches between the battery storage and the most-efficient diesel generator combination to match the load. A modem-link provides remote monitoring and control facilities.
- In Thailand, PV hybrid systems have been installed as pilot projects since 1990 (Phuangpornpitak and Kumar, 2007). Most of them were adapted for national parks and wildlife preservation areas or rural villages that do not have access to electricity. Nine off-grid PV hybrid systems ranging from 5 to 82.5 kW, with a total installed capacity of about 285 kW, are in operation and constitute about 10% of the total PV power installed in Thailand. The first hybrid power system in a wildlife sanctuary, Huai Kha Khaeng, was set up by King Mongkut’s University of Technology Thonburi in 1998 with the aim to assess technological, economic and operating aspects and to study the penetration of PV in remote and preserved areas. During 1998–2003, the system supplied 44,504 kWh (PV supplying about 88.5% of the total demand) or an average of 24 kWh/day. The PV/diesel hybrid system installed at Huai Kha Khaeng wildlife sanctuary in 1998 was optimised in order to meet an increasing demand for a clean and reliable power source. The system can supply electricity to load because the diesel generator works to compensate any inconvenience caused by photovoltaic. PV-wind-diesel hybrid systems were installed in 1999 at Phu Kradung, a high-elevation national park in Loei Province, and at Tarutao, an island in a marine national park in Satun Province, Thailand.
There could be several barriers to the implementation of hybrid technologies and these need to be overcome for a successful establishment of projects. Hybrid systems generally have a relatively high investment cost, which makes smaller projects unattractive to the investors, lenders, project developers, and manufacturers. Similarly, these technologies have several technical barriers which include: requirement of redundant generation systems, a time limitation for the generation of electricity, need for sophisticated control systems, need for storage systems, and transmission line losses.
Other aspects in the implementation chain of these hybrid technology systems in developing countries could be the limited credit worthiness for potential investors; absence of a power purchase agreement with energy users (e.g. through the grid operator); absence of energy or power systems in the villages; lack of information on market, employment, rural development and other economic information; lack of vocational education, communication availability or other social development activities; lack of human capital to properly operate the power plants; and lack of financing partners.
There are several research programmes on hybrid technologies all over the world, mainly in developed countries. For instance, Princeton Energy Resources International  (PERI) has undertaken various research programmes on wind power and other wind-based hybrid technologies. PERI has developed several databases and analysis tools to track and analyse wind system and subsystem cost, performance, and other characteristics (Princeton Energy Resources International, no date). Recent use of these has involved projections of expected technology development paths over time and evaluation of financing/ownership on both a corporate balance sheet basis by investor-owned utilities and tax-free public utilities, and a project finance basis through independent power producers.
To help facilitate adoption of wind/diesel hybrid systems, PERI has analysed the potential market for replacing existing diesel plants with wind turbines in rural Alaska (USA) for the National Renewable Energy Laboratory (Princeton Energy Resources International, no date). The objective of this assessment was to characterise the size of the wind-diesel hybrid market so that the State of Alaska and Alaskan rural electricity authorities can determine the level of effort required to develop wind projects. An initial list of about 90 Alaskan villages was identified as having outstanding wind resource potential. The result of this analysis was a ranking that identifies the villages where wind/diesel hybrids will have the most favourable economic characteristics.
During 2001, the Photovoltaic fuel cell hybrid systems (PVFC-SYS ) project was carried out as a European Commission research project on the hybrid technology (European Commission, 2001). The main aim of the project was to study and develop a low-power energy generation system, which would utilise the synergies between a photovoltaic generator and a Proton Exchange Membrane fuel cell. Such a system in the range of 5 to 10 kW is intended to be a future competitor to hybrid PV-Diesel systems, especially from an environmental point of view as emissions of both exhaust gases and noise will be drastically reduced. The overall target of the project was the development of a hybrid system based on an innovative package using hydrogen as a fuel. This can be considered a zero emission system. The use of the so-called innovative components will open new possibilities of future cost decrease, both in the investment, operational and replacement point of view. Since there are no moving parts, less maintenance is required and the lifetime of the components is expected to be higher.
In 1998, China launched an ambitious ‘Brightness Programme ’ that targeted household and village-scale applications of solar PV and wind energy in off-grid regions, particularly in western China. In 2002, the Chinese Government started a major new rural electrification initiative called the Song Dian Dao Xiang programme (National Township Electrification Programme). This programme is directed at electrifying approximately 1000 townships in seven provinces in western China with about 17 MW of village-scale hybrid systems (mainly PV, with some wind, combined with batteries and diesel back-up systems). The required funding amounts to RMB 2 billion (USD 240 million), which covers 50% of the capital costs of village power systems (in Tibet: 100%) (Martinot and Wallace, 2003).
In 2001, 70 village-scale hybrid power systems (wind and/or PV combined with battery storage and many using backup diesel generators, ranging in size from 5-200 kW) were installed in China (Martinot and Wallace, 2003). A 100-kW wind-diesel hybrid village power plant was under construction in 2002 in Zhejiang (Bei Long Dao). A second hybrid system consisting of 80 kW of wind and 20 kW of PV power became operational in Xinjiang in December of 2002.
Market studies indicate that by 2010 at least 1000 MWp of stand-alone PV hybrid systems will be installed worldwide, both for remote buildings and on islands (Lysen, 2000). In order to realise this potential and reduce the costs of these hybrid systems, still a lot of work remains to be done, for example, through standardisations and modularity and by developing proper monitoring systems to reduce maintenance costs.
Technology transfers from industrialised countries could help improve these implementation chains and demonstrate the working of the hybrid systems. The aforementioned EU research group study (European Commission, 2001) in this respect recommends to improve the reliability of systems, reduce their costs, and reduce maintenance need or make maintenance easier. In order to meet these targets, the research has focused on different aspects such as improvement of the methods and techniques to reduce cost for the wind assessment and optimisation of rotor controls along with optimisation of the overall system layouts and controls. In addition, co-operative R&D projects, co-ordinated to use the best technology from each member of the EU are required to improve the technology for all. Directing current testing facilities to develop norms and standards in their demonstration projects will help in the continued development of this market for Europe.
For the future development of the international market of the hybrid technologies in developed countries, various stakeholders must be brought together and appropriate financing modalities used to facilitate sustainable, decentralised markets for those technologies that have the attributes of fuel flexibility and hybridisation, particularly with renewable technologies. The primary challenges for organising and delivering hybrid project financing will stem from the large number of small projects, which characterise most of these rural, peri-urban and urban markets.
In developing countries, there is a necessity of creating and utilising near-term capital and targeted subsidies, reflecting the fact that hybrid systems are currently pre-commercial and not yet financially viable. The countries should develop concessional co-financing which uses commercial methods tied to commercial capacity building and conducting strategic programmes of hybrid systems.
Renewable energy sources such as wind and solar used in wind-solar hybrid systems are sustainable energy sources as they are easily and abundantly available in nature. Similarly, hydrogen, which is used in fuel cells, could be by far the most abundant fuel resource since it is part of the water molecule. Hydrogen used in fuel cells is converted to electricity, but it can also be combusted as with the space shuttle rocket boosters using liquid hydrogen. The hybrid systems with combustion turbines and fuel cells can create systems with exceptionally high efficiency with low emissions. The hybrid systems, in general, combine generation and storage technologies so that excess of electricity can be generated during optimal times while electricity is used from the storage at other times. This will help in achieving sustainability in energy for future.
In contrast to conventional power generation systems (diesel generators, coal power, natural gas combustion), renewable energy technologies can generate heat and electricity without producing GHG emissions. Utilisation of renewable energy could play an important role in reducing GHG emissions. Considering the total life cycle of the energy generation process, it has been demonstrated that wind turbines are the cleanest and green energy systems and that hydrogen based fuel cells are environmentally friendly. However, in remote communities wind or fuel cells as stand-alone systems lack reliability, but when combined they could become more reliable.
PV and fuel cells represent two very promising industries in term of employment, in particular with respect to the identificaiton and development of new applications.
A general assessment of the cost of fuel cell hybrid technology carried out by Rastler and Lemar (2002) shows that costs of any type of hybrid technology are expected to fall to USD 600 - 1100 per kW for the period beyond 2010. The US Department of Energy has made a target of reducing the cost of fuel cell turbine hybrids to USD 400/ kW by 2010 (Victor, 2003). The life-cycle cost for a wind energy hybrid system requires the estimation of the following quantities: system life, component and total capital costs per unit of outputs (e.g., wind turbine, engine generator, controls, inverter, AC/DC converter), as well as the battery storage cost per kWh, total hardware cost plus installation and indirect costs occurring (capital cost), annual operation and maintenance and fuel costs, and equipment replacement costs occurring during the system lifetime (Notton, et al., 2001). If the system is a wind PV hybrid system, then the total cost will include the investment and installation cost of solar panels.
Wind energy systems are one of the most cost-effective home-based renewable energy systems. A small turbine can cost anywhere between USD 3,000 and 35,000, depending on size, application, and service agreements with the manufacturer. According to the American Wind Energy Association (AWEA, 2001), typical home wind system costs approximately USD 32,000 (10 kW). As a general rule of thumb, the cost of a residential turbine is estimated at USD 1,000 to USD 3,000/kW. Hence, the cost of hybrid systems with wind energy systems could decrease in the near future. In Thailand, most PV hybrid systems were installed through the co-operation of King Mongkut’s University Technology Thonburi, the Provincial Electricity Authority and the Electricity Generation Authority of Thailand. The systems were funded by the Energy Policy and Planning Office, though the communities have been responsible for operation and maintenance of the systems. The costs of the systems depend on size, location, customer type and technical specification. The cost of grid-connected systems amounts to about USD 2/Watt whereas for standalone systems the costs amount to about USD 3–4/Watt (Pvresources, no date).
The Inner Mongolia Autonomous Region (IMAR) has been working in the past decade to provide stand-alone renewable power systems to rural area households: more than 120,000 households have started generating electricity with 100-300 watt wind generators (American Wind Energy Association, 2001). In the first phase of this project, the University of Delaware, the US National Renewable Energy Laboratory, and the Inner Mongolia team completed a levelised cost analysis of rural electrification options for several counties. It was found that for the output range of 200-640 kWh/yr, levelised cost of energy produced is USD 0.50-0.63/kWh. In the case of a PV system only, for the output range of 120-240 kWh/yr, the levelised cost of electricity produced would be USD 0.77-0.83/kWh. For small hybrid systems in the range of 400-750 kWh/yr, the cost amounts to USD 0.57-0.72/kWh, and for the large hybrid systems, with an output range of 560-870 kWh/yr, the costs are USD 0.43-0.57/kWh. For the types of systems currently being deployed for stand-alone electrical generation in rural areas of IMAR, wind generators are the least-cost option for household electricity (American Wind Energy Association, 2001).
The PURE (Promoting Unst Renewable Energy ) project is a pioneering project on the windswept island of Unst, the most northerly island of the UK (PURE Project, no date). PURE is a demonstration project that shows how wind power and hydrogen technology can be combined to provide the energy needs for a remote rural industrial estate. It has been developed by the Unst Partnership Ltd., a community development agency established by the Unst Community Council to support local economic development and regeneration. This is the first community-owned renewable energy project of its kind in the world and thus represents an important milestone in the development of green energy systems. The Unst Partnership, siGEN Ltd., and the Robert Gordon University, through the UK Department of Trade and Industry (DTI)’s Knowledge Transfer Partnership scheme, worked together to deliver the hydrogen system. Significant differences between the PURE project and other hydrogen energy systems deployed around the world are the scale and the low budget within which it has been developed. PURE has uniquely been developed with a comparatively small project budget of approximately €475,000 (£350,000). This budget also includes all the engineering and consultancy works surrounding the project, as well as the hardware (Hutt and Johnstone, 2005).
National Energy Technology Laboratory (NETL) and Fuel Cell Energy (FCE) are working collaboratively to do large-scale expedient testing of an atmospheric Direct FuelCell/Turbine (DFC/T) hybrid system. The R&D efforts have thus far resulted in significant progress in validating the DFC/T cycle concept. FCE has completed successful proof-of-concept testing of a DFC/T power plant based on a 250-kW DFC integrated initially with a Capstone 30 kW and then a 60 kW modified mictroturbine. The results of the system tests have accumulated over 6,800 hours of successful operation with an efficiency of 52% (Williams and Marut, 2006).
In 1995, in China, the State Development and Planning Commission (SDPC), the State Economic and Trade Commission (SETC) and the Ministry of Science and Technology (MOST) formulated a “Programme on New and Renewable Energy from 1996-2010” and launched the “Sunlight Programme ”, which will run until 2010 and which covers PV systems. It is designed to upgrade the country’s manufacturing capability of solar technologies, to establish large-scale PV and PV-hybrid village demonstration schemes, home PV projects for remote areas and to initiate grid-connected PV projects. The “Brightness Project”, which was first launched in 1996 is aimed at providing electricity from solar and wind energy in a number of remote regions (WEC, no date).
The Canadian CANMET Energy Diversification Research Laboratory (CEDRL) addresses the challenges associated to the technical needs via its PV hybrid Programme (Hybridinfo, 2001). This five-year initiative, which started in 2001, consists of R&D and technology transfer activities aimed at improving the performance and cost effectiveness of these systems, and at increasing the capacity of the solar industry to supply efficient systems.
American Wind Energy Association, 2001. Wind Energy Applications Guide. Available at: http://www.awea.org/pubs/documents/appguideformatWeb.pdf 
Dayu, Y., no date. Local Photovoltaic (PV) – Wind Hybrid Systems with Battery Storage or Grid Connection. Available at:
European Commission, 2001. Photovoltaic Fuel Cell Hybrid system for electricity and heat generation for remote sites, Community Research. Available at: ftp://ftp.cordis.europa.eu/pub/eesd/docs/ev260901_poster_pvfc-sys.pdf 
Hutt, J. and Johnstone, M., 2005. Skills for renewable energy in Scotland. Available at: http://www.ejscotland.info/skillsforrenewableenergyreport.pdf.pdf 
Hybridinfo, 2001. The bi-annual newsletter on photovoltaic hybrid systems in Canada, Canmet Energy Diversification Research Laboratory Photovoltaic Program, Issue 1. Available at: http://cetc-varennes.nrcan.gc.ca/fichier.php/codectec/En/2001-42/2001-42e.pdf 
Ken Tec Denmark, no date. Hybrid Systems, The Concept. Available at: http://www.kentec.dk/?ID=122 
Lysen, E.H., 2000. The International Energy Agency PVPS Programme - PV hybrids and the future, PV Hybrid Power Systems Conference, Aix-en-Provence, France.
Martinot, E. and Wallace, W., 2003. Case Study: UNDP/GEF Project for Commercialization of Renewable Energy in China. Available at: http://www.martinot.info/Cases/China_RE_GEF.pdf National Energy Technology Laboratory, no date
Patel, M.R., 1999. Wind and Solar Power Systems: Design, Analyses And Operation.
Phuangpornpitak, N. and Kumar, S., 2007. PV hybrid systems for rural electrification in Thailand. Renewable and Sustainable Energy Reviews, 11, pp. 1530–1543.
Solar Energy Technologies Program, 2006. Energy Efficiency and Renewable Energy, PV in Hybrid Power Systems, U.S. Department of Energy.
Available at: http://www1.eere.energy.gov/solar/hybrid_systems.html 
UNEP, 2003. Switched On - Renewable Energy Opportunities in the Tourism Industry. Available at: http://www.uneptie.org/pc/tourism/library/energy.htm 
US Climate Change Technology Program, 2005. Technology Options for the Near and Long Term. “Distributed Generation/ Fuel Cells, Technology Descriptions”, August 2005. Available at: http://www.climatetechnology.gov/library/2005/tech-options/tor2005-fullreport.pdf 
WEC, no date. WEC, no date. Energy Information, World Energy Council, China. Available at : http://www.worldenergy.org/about_wec/ 
Williams, M.C. and Marut, H.C., 2006. Distributed Generation: Molten Carbonate Fuel Cells, U.S. Department of Energy, National Energy Technology Laboratory.