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Distributed Power Systems For Sustainable Rural Development Environmental Sciences

Essay add: 14-11-2017, 16:38   /   Views: 3

This paper describes renewable energy systems, which are designed to meet the demands of rural populations that currently have no access to grid-connected electricity. It is well recognized that rural populations need safe drinking water and a centralized refrigeration system for storage of medicines and other emergency supplies. Here we propose a multi-generation district system that will generate affordable, reliable clean electricity, and supply the heat needed for both absorption refrigeration and membrane distillation (MD) water purification.

1. Introduction

In 1987, Dr. Brundtland and her commission released its report on the relationship between mankind and the state of the environment, entitled Our Common Future [] . The report defined the concept of "sustainable development" as strategies that satisfy "the needs of the present without compromising the ability of future generations to meet their own needs." Our Common Future asserted that the most important priority was to address the problems of worldwide poverty. Five years after Our Common Future was published, Rio de Janeiro hosted the United Nations Conference on Environment and Development, commonly known to as the 1992 "Earth Summit." This conference identified the various environmental challenges to achieving sustainable development. The Earth Summit called for international policies regarding conservation and pollution, as well as continued research into alternative forms of energy production.

Since the publication of Our Common Future and the proceedings of the Earth Summit, the catch-phrase "sustainable development" has been used to advocate any number of alternative energy-source scenarios, from green architecture to nuclear power. Based on the criteria established by different stakeholders, the term "sustainability" carries different meanings. For example, some would argue that replacing gasoline with biofuels for use in transportation might partly constitute a sustainable energy scenario. Others may assert that using nuclear power to generate hydrogen through electrolysis, thus providing a hydrogen economy, offers the best hope for achieving a sustainable energy transition. Successful sustainable development depends upon a secure, reliable supply of energy that is accessible and affordable to developing and developed nations alike. The environmental consequences of energy extraction and conversion may pose a more immediate threat to world security and peace than fears over permanent exhaustion of resources. Perhaps the greatest challenge facing society in the 21st century is the development and implementation of sustainable energy sources that can meet the needs of populations throughout the world while conserving the earth's life supporting systems. Some studies have indicated that a total shift towards a sustainable energy system is possible within a period of about 50 years [] . Such reports are encouraging, but it is still important to critically examine proposed sustainable energy scenarios for economics and efficiencies of the conversion process, reliability of power generation, and availability to developing nations.

When Thomas Edison built his historic Pearl Street station in 1882 to serve nearby customers, he envisioned a highly dispersed electricity system with individual businesses generating their own DC power. The bulk of his company's business was based on the construction of small "isolated" plants that were self-contained and sized as low as 100 kilowatts. By the late 1890s, many small electrical firms were doing brisk business marketing and building power plants that not only generated power, but also provided district heating and reused waste heat - now commonly referred to as cogeneration systems. The popularity of these small, independent electrical providers was such that by 1907, they were producing 59% of the electricity in the U.S. and had developed cooperatives to sell excess electricity to other users. [] 

The reemergence of small-scale electric power generation (referred to as micropower [] ) is the result of new, digital- or computer-dependent economies that place a very high premium on reliable power. A distributed or decentralized network of small systems can provide a higher quality and more reliable source of power that is essential to many high-tech industries, medical centers, and financial institutions. Thus, the 21st century idea of generating power on a smaller scale and nearer the point of use is the same as that originally envisaged by Edison in the 1880s. This model for energy production is called "distributed power," and it is the modus operandi in most discussions of sustainable energy development and renewable technologies.

Nearly 68.84% of the population (1.21 billion) in India lives in rural areas (640,867 villages) [] . Nearly 300 million lack any kind of electrical grid connection and of those who do, find electricity supply intermittent and unreliable. For example, the brunt of the electricity shortage is borne by villages with power cuts ranging from 8 -14 hours a day on an average. In these locations, where the mechanical or electric power needs are only at the level of the individual household, farm, or village, there are good opportunities for implementation of small-scale distributed renewable energy systems at competitive costs. The long lead times required to put fossil fuel based infrastructure in place are also a hindrance to development in countries like India, where energy demand is growing rapidly. Such situations may also offer favorable prospects for introducing renewable energy services under competitive conditions.

Renewable energy services now make it possible to bring modern energy technologies to the many that still live without any form of electricity. Further, a case will be made that the entire demand for energy can be met from solar energy sources and solar resources. The term "solar resources" refers to materials of plant origin produced from sunlight via photosynthesis, also called biomass. The small scale and modularity of the technologies such as photovoltaics, solar-thermal systems, and biomass gasifiers, and the fact they are clean and safe, makes them an attractive option for rapid implementation.

2. Human Development and Energy Needs

A measure of social and economic development that impacts human well-being is represented in the United Nations' "Human Development Index" (HDI), a composite index that includes data on standard of living, represented by a discounted value of GDP

per capita, educational attainment, longevity, gender-related development and empowerment, technological achievement, and other conditions that contribute to the physical, social, and economic health of countries and populations. Two-thirds of the index weights represent attributes that are not directly related to economic activity and energy use. The human development reports of the United Nations [] explain in detail how the index is computed.

Figure 1. Human Development Index (HDI) variation with per capita residential electricity consumption by country (UNDP, 2011 and IEA 2010) [] 

The HDI is extremely useful for evaluating human progress over time and for comparing the level of social resources within various populations. For example, in countries with a HDI value of 0.8 or greater the average life expectancy is 77 years, while those nations with an HDI below 0.5 have an average life expectancy of only 50.6 years. The relationship between measures of human well-being and the consumption of electricity is explored in Figure 1, in which the HDI is plotted against the annual per-capita electricity consumption for a number of countries. Overwhelming evidence suggests that small improvements in the level of energy services to people living in poverty could help to generate significant changes in their quality of life.

The UNDP data also indicates that an HDI of 0.8 or greater corresponds to electricity consumption above 1500 kWh/year, which is associated with countries having a high GDP per capita6. At the other end of the scale, where almost two-thirds of the world's population uses less than 1000 kWh per capita, a significant improvement in HDI can be achieved with marginal increases in electricity use. For example, the value of HDI for India is 0.547, which is in the Zone 2 as identified in Figure 1. The annual per capita residential energy consumption in rural and urban areas is about 96 kWh and 288 kWh respectively [] . For the residential sector (about 39% of the final energy consumption), electrical energy sales to domestic consumers amounted on average to 106 kWh in 2007/08, with a wide range from 18 kWh/cap in Bihar to 424 kWh/cap in Delhi [] .

Although the relationship between energy consumption and economic growth is now well established, the question of whether economic growth leads to energy consumption or energy consumption is the engine of economic growth remains a subject of some debate. In the case of the U.S., the unidirectional causality from GDP growth to energy consumption during the post war period (1947-1974) is a generally accepted truism. However, with efficient energy use, it can be shown that the GDP increases without corresponding increase in energy consumption. For example, between the years 1974- 2004, Sweden has increased it's GDP over 50% with an insignificant increase in total electricity consumption [] .

In the present context, it is suggested that the GDP per capita will increase with the energy consumption per capita. If economic growth occurs, the demand for energy increases as well. All attempts to meet the future energy growth should pay particular attention to the technologies that are especially conducive to implementation among the largest segment of the global population. There is a significant untapped business opportunity in providing and promoting sustainable energy services. This paper examines engineering solutions that will provide affordable, distributed power generating systems to meet the energy needs of more than 300-400 million new consumers, without dependence on government subsidies beyond the initial installation costs. Particular attention will be paid to those at the bottom of the economic scale who live mostly in rural communities. By providing the necessary energy services to the rural poor, the large-scale migration of the rural population to urban communities may be abated.

The production and use of energy also has environmental consequences to which the poor are especially vulnerable. People living in poverty are often forced to engage in hazardous or ecologically disruptive activities in order to gain access to energy sources [] . These people often use inefficient and relatively more polluting energy systems than those who are better off, as illustrated by the fact that more than 2 billion people worldwide continue to cook using traditional fuels, such as kerosene. The energy system proposed here will demonstrate that the poor can protect themselves, as well as the environment, while sustaining their livelihoods as long as they are provided access to affordable technologies. Hence, energy services, as opposed to energy consumption, will be our development indicator.

Since energy plays a substantial role in the everyday lives of humans the sustainable rural electrification program must, at the least, be capable of providing light and energy at a level that is affordable enough for income generating activities, thus helping to break the cycle of poverty and bring rural communities as full participants in the economic growth of developing nations. It is with this in mind that energy systems are proposed that use relevant technologies to harness the sun's radiation and biomass. The development of these systems require that they meet the primary objectives of being easily scalable, built with materials that are easily accessible and, lastly, simple enough for a 'bicycle mechanic' to operate and maintain.

Electricity Needs and Affordability

The electricity needs of rural poor, who often live in remote areas, are quite different from the urban and grid accessible populations. The exact need may be difficult to assess, but a rough estimate may be made using the data provided in Table 2. The information provided in the table assumes an average family size consisting of four people. The estimates given are for end-use equipment that is currently commercially available. The estimated power use is about 0.35 kWh per day per person. The annual power use per person is about 128 kWh. Hence, a small amount of power will be sufficient to make a huge impact on the quality of life. With rapid advances in efficient end-use technologies, such as LED lighting (Light Emitting Diode: 200 lumens/W) and efficient LCD TV, the daily energy usage will be further reduced. At present, the initial costs of these technologies are relatively high but declining rapidly for implementation.

Table 2. Estimated capital and energy costs for household electrical servicesEquipment(# units)Capital cost (US$)Power demand [] (kW)Usage (hours/day)Daily energy use (kWh/day)

Light fixtures (2)

6

2 Bulbs

(9W LED - 50K hour life)

20

0.018

5

0.09

TV (1774 cm2 energy star LCD color television)

100

0.041

5

0.21

Ceiling Fan

30

0.030

8

0.24

Other (Computer etc.)

250

0.05

3

0.15

Water pump

20

0.735

6.85

0.2 [] 

Cooking

50

0.2

2

0.4

Total per household

476

1.075

1.26

Total per person132

0.27

0.35

Besides the electricity need identified before, rural communities typically require power for water pumping for irrigation. For simplicity, the rice crop is considered here because of its reliance on water flooding of the fields. Ditches and canals are often used for water delivery to fields. In such circumstance, the pumps used for shallow water wells are most suitable. The most common pump for shallow well (< 7.5 m) is a jet pump, which draw the water up from the well through suction. The pump stays above ground and is powered by an electric motor that drives an impeller, or a centrifugal pump. Pumping hours for a one-inch or 2.54 cm of water on an acre of land using a 20 GPM (Gallons Per Minute) is about 22.5 hours [] . A typical one HP (0.735 kW) motor driven pump will be able satisfy the above water supply requirement. Assuming the pump will operate about 2500 hrs annually supporting about 25 families, resulting in an average electricity consumption of about 0.05 kWh/person/day.

Another essential use of the electrical energy is for cooking. The 12V DC portable stove (1.5 liter liquid capacity) is most suitable for the current application. The stove typically require about 200W startup with 120W output during cooking. These stoves maintain a cooking temperature range of 120-150oC. The energy requirement for a typical single stove is about 200 W and its expected usage will be 500 hrs annually, resulting in an average per capita electricity consumption of about 0.075 kWh/day. Very often, solar cooker is promoted for cooking purposes. Because of its use only during the sunlight hours, the usage of it is limited in rural application, where the wages are earned mostly during the daytime.

The total electricity consumption with all the energy services is estimated to be 0.35 kWh/person/day, which results in per capita electricity consumption of 128 kWh/ year. Thus, the electric power per capita would be quite small, but it would be sufficient to meet basic necessities and improve the HDI considerably.

While describing the energy problems of the poor, the International Energy Agency points out that about 1.6 billion people are living on less than $2 per day. For the purposes of the discussion here, the individual's daily income will be taken as $2.59 with an annual income of about $259 [] . It is reasonable to assume that two people per household will earn this annual minimum wage, thus resulting in an annual income of $518 per household or ($129.5/capita). Because of the lifestyle changes electricity brings, it is observed that spending about 10% of the family income towards the electricity services is quite common. The estimates of per capita income ($129.5) and average required electricity services (128 kWh) result in an affordable unit cost of electricity of about $ 0.10 per kWh or INR 5.5 per kWh.

As poverty alleviation becomes a national priority in many developing countries, such as in India, the government must take the responsibility of subsidizing the purchase of the high-efficiency end-use equipment. As an example, the government of the Tamil Nadu has recently initiated a scheme to provide free appliances (Fan, color televisions etc) to most of the rural poor. It would also be prudent government policy to provide the initial capital to build the distributed power systems in lieu of capital expenditures incurred in establishing the grid and the associated central power plants. Such an idea was implemented in Tamil Nadu under the recently announced solar-powered greenhouse scheme, which is expected to cost the government about $216 million dollars.

With the high "first cost" of the capital expenditure of the power generating system and appliances is largely met by government subsidies, the estimated kWh price of $0.11 must be able to support any operational and maintenance (O&M) expenditures.

The most promising solution to the energy problem, especially in the rural communities, appears to be the use of the Sun's radiation as an energy source. The sun pours more energy onto the Earth's surface in one hour than the entire planet uses in one year. The three logical approaches that are amenable are the direct conversion of solar radiation into electricity via solar cells, the conversion of solar radiation into heat that can then be thermodynamically converted into electricity and the use of biomass.

Commercially available solar cell prices have significantly declined to a point that an installed cost of $2/W is not unreasonable to expect currently. But there are strong indications that science will deliver new materials that will make this technology more accessible in the near future with an expected installed cost reaching $1/W [] . Most of the poor are in countries that are located in regions where annual solar radiation is available at about 2000 kWh/m2. In these regions, another possible approach is the conversion of solar radiation into heat through the use of a concentrated solar power system (CSP). The application of CSP is mostly confined to large-scale systems, typically 50 MW or higher. The recent developments in the medium temperature parabolic troughs and low cost dish-sterling systems may help bring CSP application to 100 kW to 1 MW systems. In either of these approaches considered here, the electricity is generated during sunlight hours with most of the demand being during the evening and nighttime. Hence, it is essential that efficient and economic energy storage be a part of the system. Energy storage is expensive and will raise the cost the solar energy on demand, but unavoidable in the current application.

The use of small-scale biomass as an energy source has been a mainstay for centuries, and still continues to be used in many rural areas. However, the sources of biomass are scarce in many arid areas and will only become worse with increasing population and the effects of global warming. Hence, a well-managed energy crops program must be part of the overall energy strategy in any sustainable rural development program. The photosynthesis process uses solar energy to combine carbon dioxide from the atmosphere with water (and various nutrients) from the soil to produce plant matter (biomass). Because photosynthesis captures CO2 from the air, the resulting carbon based feedstock can be processed and utilized in a similar manner to fossil fuels with lower net CO2 emissions. The naturally low efficiency conversion of solar energy to biomass leads to large requirements of land, water and nutrients. Increases in the yield of dedicated energy crops for given energy, water and nutrient input would decrease the associated life cycle costs. The carbon dioxide (CO2) emitted by high efficiency combustion or gasification of biomass is taken up by new plant growth, resulting in zero net emissions of CO2.  However, it should be remembered that there are some net CO2 emissions associated with bioenergy when looked at on a life-cycle basis - emissions from fossil fuels used in the cultivation, harvesting and transport of the biomass. These are generally small compared to the CO2 avoided by displacing fossil fuels with energy from biomass.  Consequently, bioenergy is a renewable energy resource with the added benefit of being CO2 neutral.

4. Rural Infrastructure

The power network in rural areas has been expanded through a centrally planned approach and has resulted in the creation of a vast network with 82 % of villages covered by a supply of electricity. However, the supply of power to the rural areas is erratic and its quality is poor. Thus, even for connected households, on an average power is available for 40 percent of the time. The government provided power in rural areas is managed through village-level transformers. A typical rural power line infrastructure is shown in Figure 2. Although, the existing power transmission network to 'bring power' to homes is meager, but can be easily improved with small investment to deliver the reliable power. Many field surveys suggest that with increasing income, the rural house holds has the necessary affordability to pay non-subsidized tariff rates for good quality supply.

Figure 2. A typical rural electrification infrastructure.

The successful experience with small independent electrical providers in the United States appears to be attractive to adopt in the present context3. Especially, when the low voltage (11 kV) transmission grid infrastructure already present in most villages. However, the existing infrastructure needs to be converted into local micro grid independent of the publically owned high voltage grid. The management of local generation and distribution through Panchayat institutions, users' associations, co-operative societies, non-governmental organizations or franchisees can be most effective. Very often, it is necessary to provide micro-credit to those who can afford the tariffs but not the connection costs or the appliances that run on electricity. The goal is to shift to providing more economically viable infrastructure service, using affordability criteria. This approach envisages that initial capital costs will be provided with the necessary government subsidies. The model with locally owned private ownership should result in improved service standards, increased efficiency and an economically viable small-scale industry with assured returns.

5. The Approach

The traditional approach to rural electrification has mostly been small-scale photovoltaic systems using rechargeable batteries for energy storage. Although such systems are very attractive in the initial stages they tend to falter with time due to inadequate financial support for maintenance and battery replacement. Most of these systems are largely financed by worldwide organizations, such as the World Bank, UNDP and other philanthropic efforts, and they are designed and installed by expert companies, mostly from western countries. In order to address some of the inadequacies of this approach, more recent efforts are achieving some success by approaching the problem in a more integrated fashion [] . In this paper we propose district systems that will employ different strategies. It is envisioned that the construction of small "isolated" plants that are self-contained and sized as low as 100 kilowatts will be the norm. These plants, owned and operated by individual businesses, not only to generate power, but also provide a centralized refrigeration system for storage of medicines and other emergency supplies, as well as safe drinking water.

4.1. Photovoltaic systems

Solar cells have undergone significant changes in the last 50 years since its invention in 1954. Solar power is beginning to challenge conventional energy sources in terms of mainstream acceptance. The most comprehensive indicator of the progress in PV technology is cost per watt with other attributes such as efficiency and reliability playing a role. The cost per watt of crystalline silicon modules has declined dramatically almost by a factor of 100 since the 1960's. This reduction has been a function of several factors including improved conversion efficiency, reduced material consumption, increased manufacturing scale and simplified manufacturing process. One of the main reasons being that the production-engineering infrastructure is optimized and continues to be improved for better yield. Despite the higher material cost in the thicker crystalline (silicon) substrate and its energy intensiveness of substrate manufacturing, solar cells made from crystalline materials are comparable in cost with thin film solar cells with lower efficiency. Recent research efforts are focused on cost reduction, for example by using a thinner crystalline (silicon) substrate. Because of the reasons cited above, solar cells made from crystalline silicon materials remains popular and dominate the overall market (80-90%). Recently thin film technologies have been gaining ground. Even though the efficiencies of thin film modules are poorer, their cost per watt is lower due to less material usage. In particular, cadmium telluride (CdTe) based modules have been quite successful with their market share increasing rapidly. Other commercial thin film technologies such as copper indium gallium selenide (CIGS) and thin film silicon also increased their market shares in recent years. Third generation solar photovoltaic technologies such as dye sensitized and organic solar cells have the potential to lower the levelized cost of electricity by combining thin film approaches with high efficiency concepts [] .

Equally important in the photovoltaic system implementation is the balance of the system (BOS) cost. The term 'balance of system' refers to all of the system components except the PV modules. These components frequently account for half of the system cost and most of the system maintenance. The components consist of structures, enclosures, wiring, switchgear, fuses, ground fault detectors, charge controllers, batteries, and inverters. The BOS costs are also declining because of material reductions through design and panel efficiency (more kW/m2 lowers BOS/kWh), reducing the required skill for construction (e.g. pre-fabricated structures) and optimized supply chain.

The most effective thin film CdTe based modules cost reached about $0.60/W in 2012 with a projection of it reaching $0.45 in 2015 [] . The current module efficiency is limited to about 10-12%, but is expected to reach 15% by 2015. Similar cost and efficiency goals are also expected for the CIGS thin film modules [] . The standard BOS cost per Watt is about one dollar currently and is expected to reduce to about $0.75/W by 201517. Based on the current market conditions, the best cost/efficiency ratio is obtained using crystalline silicon modules. The PV system average end customer price for 100 kW or less roof-mounted systems decreased steadily reaching about 2.199 euro/Watt ($2.9/W) at the end of 2011 in Germany [] . Based on these observations, it is suggested that an end customer price of $2/W for systems less than 100 kW is within reach and hence, this value will be used in the following discussion.

For rural electrification, the simple stand-alone PV systems are designed to operate independent of the electric utility grid and sized to supply certain DC or AC electrical loads. Since most of the use of the electricity load is during non-sunlight hours, batteries are typically used for energy storage. However, the system should also be able to function as a direct-coupled system during sunlight hours, where the DC output of a PV module or array is directly connected to a DC load such as water pumps. A PV standalone system with battery storage is depicted in Figure 3.

Figure 3. Stand-alone photovoltaic system component arrangement.

The PV only system produces energy at certain times and those times do not always equate to when the energy is needed. Therefore energy storage constitutes an important element of any solar energy system. In the photovoltaic system shown in Figure 3, the batteries become a central component of the overall system, which significantly affect the cost. The important battery parameters that affect the photovoltaic system operation and performance are the battery maintenance requirements, lifetime of the battery, available power and efficiency. An ideal battery would be able to be charged and discharged indefinitely, have high efficiency, high energy density, low-self discharge and be low cost.

The most commonly used choice is the lead-acid battery. The most advanced technology consists of modular multi-cell valve regulated cartridges arranged in parallel-series architecture that allows for easy installation and replacement. The electrolyte is in a sealed system and is immobilized. The hydrogen and oxygen recombine internally with pressure relief valves open under faulty conditions. These batteries have typically 3-10 years life as compared to 15-20 years for open cell technology with water replenishment. The capacity, depth of discharge (DoD) and the number of cycles dictate the performance of the battery. Better "solar batteries" can give around 2100 cycles at 80% DoD, at a price of about $200 per kWh.

Meanwhile, a newer generation of batteries, most notably, lithium-ion based technology are capturing the attention at a competitive price. Normally Li Ion batteries could handle much better discharge rate compared to Lead Acid batteries. Another advantage of Li Ion battery is that at higher current ratings the battery capacity would not go down like Lead Acid battery. Modern Lithium Iron Phosphate batteries give around 3000 cycles at 80% DoD. With recent scientific advances in Li-Ion battery technology [] , it is now possible to obtain an energy density of 400 Wh/kg at a price of about $125/kWh with enhanced cycle life. Because of higher performance, lower cost and better environmental attributes, the Li-ion batteries will become dominant storage medium for the stand-alone PV system application. Hence, for levelized cost of energy calculations, the $125/kWh price will be used.

The PV power plant levelized cost of energy (LCOE) will dependent up on three primary variables; the capital cost, capacity factor (net actual generation/(period hours x net maximum capacity)) and cost of capital. Using the costs associated with the PV modules, BOS and the battery storage, the levelized cost of energy (LCOE) is calculated using a simple formula as follows [] :

CRF: Capital Recovery Factor; KI: total investment of the plant; KOM: annual operation and maintenance costs; E: annual net electricity; kd: debt interest rate; n: depreciation period in years (~25); ki: annual insurance rate (~1%).

The many factors that determine the LCOE vary greatly due to government subsidies, tax incentives and annual net electricity production. One of the key parameters in the above formula is the determination of the annual electricity generation, which depends largely on the available solar radiation at the plant location. For the present calculation, the average solar insolation in India is estimated to be about 5.2 kWh/m2/day. The annual net electricity generated is dependant upon the capacity factor (= kWh/(kWx24x365)), which is calculated using sunlight hours and solar insolation. For India, the average capacity factor is about 0.18 and the value of E is found it to be 1577 kWh per one kW installation. The annual operation and maintenance include the cost of the labor (one full time employee) and the battery replacement (twice during the life of the plant, taken here as 25 years).

As an example, a 100 kW system consisting of 400 kWh capacity battery storage is considered. Based on the current prices ($2/W for PV system and $125/kWh for battery storage), the total investment of the plant is about $250,000. The depreciation period is taken as 25 years with a prevailing simple interest rate of 15%. The annual insurance rate of 1% is assumed. The annual operation and maintenance cost is estimated to be about 3% of the initial capital cost. With these considerations, the levelized cost of energy is $0.28 (INR 14) per kWh, a value significantly higher than the affordable unit cost of $0.10 per kWh. Hence, it is imperative that a substantial (~75%) subsidy of the capital cost needs to be provided. Since, the system provides sufficient (10 hrs/day) uninterrupted power to about 400 families, the subsidy will amount to about $469 per family. Such a subsidy is in accordance with the costs associated with a traditional grid based electricity connection.

4.2 Hybrid solar/biomass system

Three quarters of Indians live in water-stressed regions. The situation is worsening with growing demand and inefficient water usage while the availability of clean water is declining due to overexploitation of groundwater and pollution of water bodies. An opportunity exists with a well-designed hybrid power plant, which concomitantly generate electricity and provide clean water while improving the overall plant efficiency. Such a system is proposed, as shown in Figure 3 and described in this section.

Biomass remains the primary energy source for the rural population in India with wood fuels being the dominant source. The unaffordability of commercial fuels in rural areas has sustained the growing biomass primarily for cooking and water heating. The increasing pressure on existing forests has already led to considerable deforestation. The sustainable growth of biomass in India therefore would require biomass resources generated from commercial energy plantations in varied climatic conditions. For electricity generation, two dominant technologies are direct combustion and gasification. Biomass fired power generation has been widely used and gaseous emissions formed in the combustion process have been under scrutiny. Pollutants produced include ash, nitrogen, sulphur etc. and CO and CxHy from incomplete combustion. Many of the pollution problems can be avoided by low emission combustion (biomass burned fast and at high temperature) at a considerable cost. To mitigate these problems, thermochemical conversion of biomass by gasification is chosen for the current application.

Gasification is a well-proven technology that has been employed in various forms for almost 200 years [] . It is essentially an oxygen limited thermochemical conversion of carbonaceous material to a useable gaseous fuel, synthesis gas or "syngas", consisting primarily of hydrogen (H2) and carbon monoxide (CO), with lesser amounts of carbondioxide (CO2), methane (CH4), higher hydrocarbons (C2+), water (H2O) and nitrogen (N2). The oxidant used can be air, pure oxygen or steam. Such a gas is used as a fuel for internal combustion engine/generator that produces electricity. For economic reasons, air based downdraft gasifier will be used [] . The waste heat output from the internal combustion engine has sufficient energy to heat water to about 80oC. In the present configuration, an air gap Membrane Distillation (MD) technology will be used to produce drinking water from a variety of sources including seawater to chemically contaminated, such as arsenic, ground water [] . A brief description of this technology is given later in this section.

The hybrid solar/biomass system is designed to generate about 172,500 kWh, the same amount of annual electricity production envisaged for the photovoltaic system considered in the previous section. Since, most of the electricity need is during the evening hours, it is proposed that the biomass gasification system will operate from 5pm to 11 pm meeting most of the demand. While the PV system will be sized to charge the battery system and provide nominal DC load during the daylight hours. A 50 kW electric generation biomass downdraft gasification system is considered. The biomass consumption of such a system is about 75kg/hr. The annual electricity production will be about 109,500 kWh with the remaining balance of about 63,000 kWh is generated by the PV system. Accordingly, a 50 kW PV system with 200 kWh battery storage will be incorporated.

The capital cost of the Biomass gasification system will be approximately $75,000. The cost of PV system with the battery storage is estimated to be $125,000. The annual biomass consumption is about 165 tones at a prevailing cost of $50/ton resulting in yearly expenditure of $8250. The depreciation period is taken as 25 years with a prevailing simple interest rate of 15%. The annual insurance rate of 1% is assumed. The annual operation and maintenance cost, excluding the biomass expenditure, is estimated to be about 3% of the initial capital cost. With these considerations, the levelized cost of energy is $0.28 (INR 14) per kWh, a value significantly higher than the affordable unit cost of $0.11 per kWh.

The 100 kW photovoltaic system considered here will meet the needs of about 400 households with uninterrupted electricity supply for over 10 hrs a day, especially during the non-daylight hours. The initial capital cost subsidy required to make the electricity affordable is within the range of subsidies considered for a variety of government and CSR (Corporate Social Responsibility) programs in rural India.

4.3. Microgrids

Microgrids are basically self-contained electrical ecosystems. Power is produced, transmitted, consumed, monitored, and managed all on a local scale. In many cases, they can be integrated into larger, central grids, but their defining characteristic is that they can operate independently if disconnected from the whole. By drastically shortening the difference between where energy generated and where it is being used, microgrids eliminate the need for heavy-duty transmission infrastructure, and thus reduce the amount of energy simply being lost along the way.

Article name: Distributed Power Systems For Sustainable Rural Development Environmental Sciences essay, research paper, dissertation