HYBRID APPROACH FOR DESIGNING SUSTAINABLE POWER GENERATION SYSTEM IN OFF-GRID ISLAND AREA

HYBRID APPROACH FOR DESIGNING SUSTAINABLE POWER GENERATION SYSTEM IN OFF-GRID ISLAND AREA Saiful ISLAM 1, Johan DRIESEN, Ronnie BELMANS Katholieke Universiteit Leuven, Belgium ABSTRACT For power generation in off-grid island area, hybrid approach with micro-grids is a good solution. Using a combination of photovoltaic modules, micro-hydro, solar thermal, diesel generator, storage devices and power conditioning systems can develop hybrid power system. The work as a whole is included resource assessment techniques, load analysis for an assumed load curve and optimisation of the different components by computer simulation/modelling. As peak load can and often does coincide with poor solar radiation, optimisation of the system involves load management and/or sizing of the energy storage. However, the choice of the components is determined by technical performance point of view as well as economic consideration. I. INTRODUCTION AND BACKGROUND For power generation in off-grid island area, hybrid approach with micro-grids is a good solution. Using a combination of photovoltaic modules, solar thermal, micro-hydro, diesel generator, storage devices and power conditioning systems can develop hybrid power system. Hybrid systems generally offer operational ease and lower life cycle costs, while maintaining optimal loading of the generators by storing energy in batteries. They are independent of a large centralized electricity grid, incorporate more than one type of power source, and are useful for remote and island areas. To maximize the use of the renewable resource, the size and operation of the hybrid system components need to be matched to the load and the available renewable resource. The research work as a whole is included resource assessment techniques, load analysis for an assumed load curve and optimization of the different components by computer simulation/modelling. INSEL simulation programme [1] has been used for the simulation. A sensitivity analysis is conducted for various up-to-date types of components with regard to different economic boundary conditions including BOS cost. As peak load can and often does coincide with poor solar radiation, optimization of the system involves load management and/or sizing of the energy storage. In this work, hybrid system has been developed for a two-storeyed guesthouse building, which will be used as a resting place for the visitors. The objective of the project is to make the building an energy island. The project site is assumed to situate in an area of high altitude with extreme climatic condition. By matching analysis between supply and demand, it is found that the RETs options for the site are needed mainly for electricity and thermal load from several renewable resources. Solar and micro-hydro resources are potentially available in the site. It could be a standalone or hybrid system. In order to provide permanent and sustainable energy generation, the usage of hybrid systems could be encouraged in the chosen site. Hybrid system will run with a diesel generator (genset) as back-up system. II. RESOURCE ASSESSMENT 1 Katholieke Universiteit Leuven, ESAT-ELECTA, Kasteelpark Arenberg 1, B-31 Leuven, Belgium, Phone: +32-16-32 1835, Fax: +32-16-32 19 85, E-mail: sislam@esat.kuleuven.ac.be

General procedures of resource assessment have been followed to estimate the energy potential from different renewable energy sources. The results of the investigations are shown below: II.1 Solar Resource Solar radiation is required in modelling the performance of PV systems and Solar Thermal. Hourly average monthly irradiation and ambient temperature shown in fig.1 have been used for the designing, sizing, simulation and analysis of the solar thermal and solar electric systems. This radiation data is extracted from MTM database (as given in the MTM block of the simulation program used INSEL)[1]. Latitude and Longitude of the site have been chosen as 47,6 North and 11,68 East respectively. For solar resources assessment, shadowing effect has Radiation and Tamb 3 2 1-1 -2-3 Jan Mar May Solar Radiation W/m2 Jul Months Sep Nov Tamb (deg C)*2 Fig.1: Hourly averaged monthly irradiation and ambient temperature data over time (yearly mean value: 159W/m2 and -2.5 deg C) been taken into account. Simulation programme SunOrb (version 1.2) [2] has been used to find out the elevation of the sun viewed from the South axis (East =-9 ; West =9 ) and then shadow map has been obtained by superimposing elevation map to sun path diagram. Finally diffuse radiation has been taken as global radiation for the shadowing period of the year. Assumed obstacles and corresponding shadow map have been shown in fig.2 and fig.3 respectfully. Angle -1 5 25 2 15 1 5-9 -75-6 -45-3 -15-5 15 3 45 6 75 9 15-1 -15-2 South Fig.2: Elevation Angle versus azimuth Angle Fig. 3: Shadow map obtained by superimposing elevation map to sun path diagram II.2 Hydropower Resource Some parameters need to be known for hydropower resource assessment. Hence, available head, depths of the river, effective cross-sectional area and the water speed throughout the year have been assumed. For the calculations, static head is assumed as 1m and flow speed (centre) and height over the year are presented in the fig.4. Calculated monthly net power and available energy have been shown in fig.5. Water height(cm) and speed(dm/s) 1 8 6 4 2 Jan Feb Mar Apr May Jun Jul Aug Sep Months of Year Oct Nov Dec Power(kW) and Energy(kWh/day)/1 1 8 6 4 2 Jan Feb Mar Apr May Jun Jul Aug Sep Months of year Oct Nov Dec Water Heights in cm Flow Speed (center) in dm/s Fig.4: Monthly flow speeds and heights of the river Pgross (kw) Pnet (kw) Energy [kwh /day]/1 Fig.5: Monthly net power and energy available

Gross power and net power potential have been calculated from the formulas given below. Overall efficiency is taken as.5[3]. Formulas used for the calculations: Flow rate, Q = V*A, V-flow speed at the center of the river [m/s], A- Cross sectional area Gross power, P gross = Q*H*ρ*g, Q- Flow rate [m 3 /s], H- Head [m], ρ- Water density [Kg/m 3 ], g- Acceleration due to gravity [9.8m/s 2 ] Net power, P net = P gross *.5, overall efficiency=.5 III. DEMAND ANALYSIS Load estimation is done by assuming that 3 to 35 people will be available during the early summer and 6 to 7 people will be available during the summer and there will be no visitors during winter in the guesthouse. Light, entertainment, communication facility, fire alarm, comfortable indoors climate all will be available. Wastewater treatment plant will be an added interest. Electrical loads for daily usage are calculated both for the month of March and September. It is found that total electrical load is 15.248 kwh/day in March and 24.9 kwh/day in September and shown in fig.6. Electrical (kw) and Thermal [(kw)/1] Load 3.5 3 2.5 2 1.5 1.5 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 Hours of Day Electrical Load March (kw) Thermal Load March (kw)/1 Electrical Load Sept (kw) Thermal Load Sept (kw)/1 Fig.6: Electrical and thermal load profile in the month of March and September Thermal load required for heating the building is calculated by finding out the heat transfer to the environment from the building and balancing internal heat gains as human releases thermal power. The main modes of heat transfer are conduction across the walls and heat carried out by air leakage from the building. Heat losses are calculated for each room separately and are then added together to get the total heat transfer from the building. For calculating the heat transfer across the walls by conduction, air temperatures on all the walls are used. This has been calculated using the solar irradiation values on each of the different walls. Theses values are obtained as a result of the simulation using INSEL. Two air changes per hour are assumed. Average thermal conductivity for the building walls is taken as.2 W/m²/K[6]. The solar gain of the windows is calculated. In addition to this it is assumed that each person produces heat of 1 W and the total heat gain from this is calculated assuming the number of person-hours present in the building. Also the thermal loads generated by other equipment including electrical devices are included in the calculations. All the calculations are done for the month of March and September. The results are presented in the fig.6. Heat Loss as a result of air exchange as well as through walls and windows and heat gain is due to stove use, human presence, solar gain and electrical energy dissipation from equipments. The difference between loss and gain is presented as heat load in the same figure. In demand analysis, energy required for purifing supply water and waste water treatment are also included. IV. COMPONENTS SIZING AND PERFORMANCE SIMULATIONS IV.1 Micro Hydro Power System After obtaining the daily load pattern for the designing months (March and September) it is compared with the net power and found that MHP is able to cover the electrical load in all the months when the guesthouse is open except in March.The assumption is to expect a demand of 15.248 kwh/day in March, April, May and October (the guesthouse is open just for the weekends) and 24.9 kwh/day in June, July, August and September (the guesthouse is open seven days a week).

FLow-Durarion Curve FLow (m3/s).8.7.6.5.4.3.2.1 Energy Potential and Extra Energy, kwh/day 1 8 6 4 2-2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Months 1 2 3 4 5 6 7 8 9 1 Percent Time Equalled or Exceeded (%) Series1 Series2 Fig.7: Flow-duration curve Fig.8: Available energy after covering demand Fig.8 shows that 7.8 kwh/day has to be met for twelve days in March which is supported by flowduration curve (fig.7)(as the guesthouse will remain open three days at the weekend in march). To achieve this, batteries can be used. Batteries will need to supply 21.24 kwh for the weekend. Assuming a 24-volt system and a loss factor of 1.2, a battery bank of 1Ah is needed, which is available in the market and would be a good solution. After deciding on the batteries to cover the required energy demand in March, the turbine could be selected. Based on the head of 1m and the flow rate of the river (min..69m 3 /sec, max..754m 3 /sec) a Pelton turbine can be selected from the fig.9..73 C/work turb bat.95.9.8.85.96.9 ld pipe generator Inv/reg/cables.5 Fig.9: Turbine selection based on head and flow rate. [3] Fig.1: System efficiencies Pelton turbine must cover the September demand after taking into account all the efficiencies. Overall efficiency is taken as.5, and breakdown of the overall efficiency is shown in fig.1. Fig.11: Pelton turbine efficiency [3] In order to cover the load in September, the turbine needs to deliver a rated power of 1.5 kw, as the efficiency from the turbine output to the load is.73. The turbine will run 24 hours at Q max in September. Flow rate in March is 3% of the flow rate in September, hence the reduction of the efficiency for this month is obvious. Fig.11 shows a relative efficiency of.95 when the flow rate is 3% of the Qmax, which leads to a drop in turbine efficiency from.8 to.76 and a drop in the system efficiency from.5 to.48. With this new value for the overall efficiency in March, battery must cover 22 kwh instead of 21.24 kwh.

IV.2 Photovoltaic System Photovoltaic system is designed to meet the whole electrical load for the worst case (September), which is assumed as 24.9 kwh/day and the subsequent solar radiation (3.87 kwh/m²/day) has been taken into account. For the sizing of the photovoltaic panel and battery bank, the following formulas and assumptions have been taken into account and the results are shown in Table1. PV sizing: DC Energy = AC Energy / Inverter Efficiency Area PV = DC Total Energy [kwh/day] / Radiation [kwh/m²/day] / System Efficiency Number of modules = Area PV / Area PV module N module in series = Nominal system voltage / module voltage at mpp N module in parallel = Total n modules / N modules in series Current in array [A] = N module in parallel x module current at mpp [A] li Cable size, S = 2 where S = cross-section of the cable, l = cable length, I = current of Vρ the PV-panel, V = voltage drop in the cable and ρ = specific resistance of the cable. Battery Sizing: Total Ah per day = [Total Energy DC / System Nominal Voltage] x System Loss Battery Capacity [Ah] = Total daily Ah per day x storage days / max depth of discharge N of Cell in series = Nominal System Voltage / Nominal Cell Voltage Table 1 Sizing of PV and Battery bank. PV Battery System Nominal Voltage 2 V Total daily Ah requirement 162 Area PV 14 m 2 Storage Days 3 Module Voltage 18V Max depth of Discharge 6% Nos. Of module in series 11 Required Battery Capacity 81 Ah Nos. Of module in Parallel 26 Capacity of selected battery 8 Ah Length of the Cable 2 m Nominal system Voltage 2 V Section Area of the Cable 16 mm 2 Nominal cell Voltage 2 V Operating Current per Module 4 A Nos. Of cell in series 1 Current in Array 14 A Voltage loss in Cables 5 V Percent Loss in the Cables 2% For the PV sizing calculations, inverter efficiency and overall PV system efficiency have been taken as 92% and 5% respectively. It is also assumed that the area of a PV module (5Wp) is.5 m². And for battery sizing 2% system loss has been taken into account for safety reasons. IV.3 Biomass Heating System Due to very harsh climate in the project site, the source of biomass (vegetation) energy is scarce. So calculations have been carried out considering only human and kitchen waste and energy output from these sources is negligible in compared to the energy demand. Hence biomass heating, which will use wooden pellets as fuel is considered as a viable option. In this project, worst case thermal load is 45 kwh/day in March. To fulfil the load demand, maximum 1 kg fuel is needed considering energy content of the fuel is 5 kwh/kg [4] and volume is 65 kg/m 3 with an overall efficiency of 9%. For this, an oversized system of 4kW has been chosen because it will perform well to match of the future energy demand and the fluctuation of daily energy supply by other sources. IV.4 Solar Thermal Collector System The performance of the solar collector is determined by the Hottel-Whillier (HW) model [5]. Rearranging the model parameters the characteristic equation can be written as:

' ' Tf Ta η = F (τα) F U L G Where, F`: absorber efficiency, τ: transmitivity of glass cover, α: absorbtivity of plate, U L : overall heat loss coefficient (W/m 2. K), Tf: fluid temperature, Ta: ambient temperature, G: radiation level (W/m 2 ). For a typical vacuum tube solar collector (1.5x.9m, 6 tubes), F`: 86%, τα:,8 and U L : 3W/m 2. K [6]. Tf Ta Hence, η =.68 2. 58 G Based on this characteristic, calculations have been carried out to find the hot water volume that 1 m 2 solar collector can generate at the required output temperature of 15 C, 35 C and 65 C under the radiation and ambient temperature level corresponding to the geographic location. The thermal output of 35 deg C for one square meter collector for a typical day in September and March is shown in fig.12.,3,25,2 kwh,15,1,5 2 4 6 8 1 12 14 16 18 2 22 24 Hours Fig.12: Simulation of collector (1m 2 ) output at a typical day in March and September Sept March Fig.13: Monthly energy output of 8m² of a vacuum tube collector at 5 C in winter term and 35 C in summer term So if the entire thermal load is supplied by solar collector the amount of solar collector would become a big number which is unacceptable due to space and economic reason. The next step therefore should be to maximize the output generated by this 8m² solar collector. The shortage will be supplied by a pellet heating system. Whatever the output is generated during winter all will be used for space heating to keep the house at some degrees above the ambient temperature. While in the summer output will be used for space heating at a comfortable level. Therefore the output temperature of the solar collector should be organized in a way that March to October output will be in 35 deg C and rest of the time 5 deg C. Fig. 13 shows the output variations of 8 m 2 collector following the above mentioned strategy. In total the 8m² of collector will be able to deliver 4164 kwh to the guesthouse. Output, kwh 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 11 12 Months of year V. LOAD MATCHING PROPOSAL FOR HYBRID SYSTEM Load fluctuates mainly in three seasons: winter, beginning of summer (March) and summer. Hence three scenarios are proposed to meet the typical loads. During winter, no electrical load is required but thermal load exists as the equipment and furniture inside the guesthouse need to be kept in a proper environment. For this, combinations of PV and solar collector have been considered. As 1 m 2 of solar collector needs 1Wp installed capacity of pump [6], 8 m 2 out of 9 available roof area (assumed) are reserved for solar collector, the remaining 1 m 2 is for PV (1 kwp) to supply electricity for the pump and will ensure that whenever heat is generated from the solar collector, electricity will be available to pump that heat away. In contrast to winter, the guesthouse is opened from March. Hence both electrical and thermal load are required in this period. Therefore special

design is needed. An investigation from the previous chapter shows that a combined solar collector and biomass heating system will together guarantee that heat demand. Investigation from previous chapters shows that micro-hydro and a supplementary supply from other source is needed to meet electrical demand. A 2 kw genset has been chosen accordingly. To reduce the cost of power mixing, big loads like washing machine, dishwasher are recommended to connect separately to the genset. In addition to these two power supplies, the electricity production is also supplemented by the redundant power from PV through the inverter. In summer, basically the supply configuration is unchanged. However, genset now is operated in emergency situations only, as the power from hydro can meet the full demand. PV is needed only to keep the pump of solar thermal system active through out the year. Otherwise, thermal system may fail due to temperature difference specifically during winter. Although solar thermal pump will get enough power to be run during summer, but in winter all other power sources will remain turn off, hence PV system is essential thus increases costs. At any case, PV will empower the grid. As micro hydro, supplying AC power, is the main source of power generation, hence it is intended to construct AC grid. Hence a 9 Wp inverter (as PV is 1kWp, and 9% undersized)[7] is needed which will be protected by switching devices. VI. ECONOMIC SITUATION For economic evaluation, the levelised cost per kwh of electric and thermal energy from the proposed hybrid system has been calculated by the software Investment Evaluator (version 1.3). Component prices, life expectancies, operation and maintenance costs are included for these calculations but it excludes any subsidy that could be asked to the energy renewable policy in some countries. Cost breakdown of electrical and thermal systems are shown separately in fig.14 and fig.15 respectively. The total electric load required of 387 kwh/year and the total thermal load required of 67,996 kwh/year has been used for this calculation. Assuming 8% for the interest rate and following the net present value calculations for the analysis, two levelised costs for implementing a hybrid system have been found: the electrical energy is equivalent to.75 Euro/ kwh and the other one, thermal energy is equivalent to.16 Euro/ kwh. 16% 21% 11% 73% 79% PV Diesel Micro hydro Solar Thermal Biomass Heating Fig. 14: Cost breakdown of electrical system Fig.15: Cost breakdown of thermal system VII. CONCLUSION The preferred option for electrification in urban areas is connection to the main grid. However, the costs of grid extension are prohibitively expensive for remote area. That s why investigation has been carried out for different stand-alone renewable energy technologies, such as PV, solar thermal, biomass heating and micro-hydro as well as non-renewable source like diesel generator. However, more convenient system may be hybrid one with a mini-grid. This is a small electricity grid system,

which connects at the users end to the loads, and which is powered by mainly micro-hydro as well as PV and in special cases a diesel generator. Solar thermal and biomass heating will meet the total thermal load. For this purpose a mini- thermal grid can be installed. A solar thermal collector and a biomass heating system will power this grid. Fig.16 shows the whole grid together (see annex.1). VIII. REFERENCES 1. Integrated Simulation Environment and a graphical programming Language developed by the University of Oldenburg, Germany. 2. SunOrb, NES (Department for Nuclear and New Energy Systems), Ruhr-Universitat Bochum, Germany. 3. A Harvey and A Brown, Micro-hydro Design Manual, ITDG Publishing, 1992 4. David Pimentel, Handbook of Energy Utilization in Agriculture, CRC Press, Inc. 198, Page 15. 5. Hottel H.C. and Woertz B. B., (1942), The performance of flat-plate solar heat collectors. Trans. of ASME 64(Feb.), 98. 6. Duffie, J.A. and Beckman, W.A., 1991.. In: Solar Engineering of Thermal Processes (2nd ed.), Wiley Interscience, New York 7. Saiful Islam, Achim Woyte, Ronnie Belmans and Johan Nijs, UNDERSIZING INVERTERS FOR GRID CONNECTION WHAT IS THE OPTIMUM?, PV in Europe Conference and Exhibition, Rome, Italy, October 7-11, 22 ACKNOWLEDGEMENT Writter is thankful to PPRE-batch 2-21, University of Oldenburg, Germeny as valuable information has been received from them to complete the work. ANNEX.1: Fig. 16: Proposed mini-grids of the hybrid system. Electric (left) and thermal (right)