Electrical performance evaluation of Johansson biomass gasifier system coupled to a 150 KVA generator

Renewable Energy 71 (2014) 695e700 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Electrical performance evaluation of Johansson biomass gasifier system coupled to a 150 KVA generator Nwabunwanne Nwokolo a, b, *, Sampson Mamphweli a, Edson Meyer a, Stephen Tangwe a a b University of Fort Hare, Institute of Technology, P/Bag X1314, Alice 5700, South Africa University of Fort Hare, Physics Department, P/Bag X1314, Alice 5700, South Africa a r t i c l e i n f o a b s t r a c t Article history: Received November 2013 Accepted 12 June 2014 Available online The economic development of any community or society at large is directly linked to energy availability and usage Concern for climate change due to intense use of fossil fuel for energy production has increased interest in alternative energy technologies such as biomass gasification A Johansson biomass gasifier system at Melani village in Eastern Cape South Africa was installed to assess the viability of biomass gasification for energy production in South Africa This system utilizes chunks of wood coming from a sawmill industry located nearby, which produces large quantities of biomass waste that pose a challenge in terms of disposal A study on the implementation of the latter gasification project has been carried out Therefore this present study aims at evaluating the performance of the system when operated on a full electrical load A custom-built gas and temperature profiling system was used to measure the gas profiles from which the gas heating value was calculated A measuring balance/scale was used to measure the quantity of wood fed into the gasifier A dummy load bank was constructed using 12 kW water heating elements connected such that they draw maximum power from each of the three phases A power meter was used to measure the current, voltage, power as well as energy from the generator during operation A cold gas efficiency of 88.11% was obtained and the overall efficiency from feedstock to electrical power was found to be 20.5% at a specific consumption rate of 1.075 kg/kWh © 2014 Elsevier Ltd All rights reserved Keywords: Gasification Downdraft gasifier Electrical performance Conversion efficiency Introduction Biomass is an organic material that stores solar energy via photosynthesis and in turn creates a source of energy in form of carbon, hydrogen and oxygen compound It contains less carbon but more oxygen and a lower heating value than the conventional fossil fuel [1] Conversion of biomass into other forms of energy such as electrical energy and heat energy is a promising alternative to fossil fuel due to its renewable nature and availability Two major conversion processes (conversion to power, heat, transportation fuel, chemicals and methane gas) exist but the choice of any is dependent on the end use application, environmental impact, economic factor, the type and properties of the biomass [2,3] These processes are thermochemical conversion and Biochemical conversion Biochemical conversion involves the fermentation of plant material with the use of yeast or genetically modified microorganisms to * Corresponding author University of Fort Hare, Institute of Technology, P/Bag X1314, Alice 5700, South Africa Tel.: þ27 833433195 E-mail addresses: nnwokolo@ufh.ac.za, nwokolonwanne@yahoo.com (N Nwokolo) http://dx.doi.org/10.1016/j.renene.2014.06.018 0960-1481/© 2014 Elsevier Ltd All rights reserved produce ethanol and anaerobic digestion of plant material to produce methane gas Thermochemical conversion involves the thermal conversion of biomass material at different temperatures and oxygen environment It includes pyrolysis, gasification and combustion This research is focused on biomass gasification, which is the thermal conversion of carboneous material in a controlled oxygen, air or steam environment to yield a mixture of gases known as producer gas and usually referred to as syngas This thermochemical process takes place in a gasifier/reactor which comes in different designs namely; downdraft, updraft and cross draft (fixed bed), bubbling, circulating and twin bed (fluidized bed), coaxial and opposed jet (entrained flow) [4] The advantages and disadvantages of these types of gasifiers are well known and documented [5,6] The downdraft gasifier has an advantage of producing gas with low tar concentration, which makes it suitable for operating gas engines and turbines used for electricity generation The concentration of tar in the producer gas generated from a downdraft gasifier is relatively low when compared to that from updraft gasifier It ranges between 10 and 100 g/Nm3 for downdraft gasifier and 50 and 500 g/Nm3 for updraft gasifier [7] Downdraft gasifiers are mostly used due to the earlier mentioned advantage Dogru et al [8] investigated the gasification characteristics of hazelnut 696 N Nwokolo et al / Renewable Energy 71 (2014) 695e700 shell using a pilot scale downdraft gasifier with kW electrical outputs An experimental study has been carried out by Sharma [4] on a 75 kW th downdraft gasifier system The temperature profile was obtained along side with gas composition, calorific value and pressure drop across the porous gasifier bed Jayah et al [9] calibrated a computer program from an experimental result in order to investigate the impact of some operating and design parameters on conversion efficiency of a downdraft gasifier The operating parameters were moisture content, particle size and inlet air temperature and the design parameters were throat angle and heat loss The aim of this study is to analyze the performance of a 150 kVA Johansson biomass gasifier system when operated on a full load bank The load bank comprises of 12 kW heating elements connected in parallel so as to draw maximum power from the generator on each of the three phases These exothermic reactions then provide the necessary heat that drives the other processes For instance, in the reduction zone, the extent to which the endothermic reactions represented by Eqs (3)e(4) occur depends on the quantity of heat it receives from the oxidation zone Furthermore this heat is used to drive off the moisture present in the wood at the drying zone [10,11] C þ CO2 2CO þ 172 kJ=mol C þ H2 O CO þ H2 C þ 2H2 CH4 (3) þ 131 kJ=mol þ 74 kJ=mol (4) (5) The gas emanating from this zone (reduction) then goes through the gas purification system consisting of the cyclone, gas scrubber/ cooler, particle interference/sawdust filters as well as a Donaldson mm paper filter Description of a Johansson biomass gasifier system 2.1 Purification units Fig represnts the different components of Johansson biomass gasifier system This system comprises of a reactor/gasifier, cyclone, scrubber, sawdust filter, safety filter, condensate tank and electrical generator The reactor consists of four zones corresponding to the four gasification processes, namely: drying, pyrolysis, oxidation and reduction The feedstock (pine wood) is fed into the gasifier hopper through the lid using an electrically controlled winch Air containing oxygen and some non-reactive gases such as nitrogen is blown into the oxidation zone through the air nozzles to start the combustion process A kW centrifugal blower is used to simulate the engine suction when igniting the gasifier The gasifier is then ignited by inserting two or three party sparklers fitted in a sparkler holder through the ignition sleeve Once combustion has started the oxygen content of the air reacts with solid carbonized fuel and hydrogen in the fuel as represented in Eqs (1) and (2) to produce carbon dioxide and steam respectively C þ O2 / CO2 12O = H2 þ À 394 kJ=mol / H2 O (1) À 284 kJ=mol (2) 2.1.1 Cyclone The raw gas exiting through the bottom of the reactor first goes through the cyclone in a tangential manner Here about 80% of the coarse carbon particles and soot embedded in the raw gas are removed through centrifugal and inertia forces and exit through a pipe sealed by a rotary valve The centrifugal force causes the particles to collide with the outer wall while moving downwards with the gas flow through inertia At the bottom of the cyclone the gas flow reverses its direction and begins to move up It then exits through a vortex finder at the top of the cyclone while the particles exit through the bottom The particle collection efficiency of the cyclone depends on the size of the particles and the design of the cyclone as they come in different designs [11] 2.1.2 Gas scrubber/cooler The hot gas from the cyclone enters the scrubber through the bottom at a temperature of about 500  C and exits through the top at room temperature (25  C) This loss of heat is undesirable in most applications, especially where the heat from the gas can be recycled and reused [20] In addition the scrubber removes the remaining Gasifier Electrical generator Cyclone Gas Scrubber Sawdust filter Pump Cooling pond Fig Schematic diagram of Johansson biomass Gasifier Safety Filter N Nwokolo et al / Renewable Energy 71 (2014) 695e700 fine carbon particles and soot in the gases that pass through the cyclone This process washes off about 0.8 g/m3 of gas, which is translated to about 20% of the fine carbon particles These particles (those less than 0.1 mm) are collected by diffusion when water is sprayed from the top of the scrubber while particles greater than mm settle by gravity and are collected gravitationally, by impaction or by centrifugal means [11] 2.1.3 Sawdust and safety filter/paper filter The sawdust filter acts as a barrier and captures the very fine carbon particles that exeunt with the gas through the scrubber The sawdust filters are filled with very fine sawdust that collects particles through adsorption Lastly the clean gas goes through the safety filter, a double cartridge Donaldson air tight filter with a special gas tight seal between the dust bowl and the body of the filter 697 WhereCOvol, H2vol and CH4vol represents the volume concentration of carbon monoxide, hydrogen and methane present in the producer gas respectively COHV, H2HV and CH4HV represent the heating value of these gases as stated in the standard gas table The electrical performance of the generator was measured using a portable energy meter which is capable of providing the load profiles of the generator phases when powering the load bank The energy meter recorded all the energy parameter from the three phase generator at a preset interval of The recorded data usually presented either in a graphical or statistical formats was downloaded into the computer via the powerTrack software for analysis This power track software serves as an interface between the computer and the energy meter, it allows communication to exist between the two devices 3.2 Mass balance/energy balance and efficiency determination 2.1.4 Electrical generator The electric power generator is a self excited three phase synchronous generator equipped with an automatic voltage regulator This is an internal combustion gas engine, which was formerly operated on diesel but modified to operate on a 100% producer gas emanating from the gasifier The three phase alternator coupled to the producer gas engine has a capacity of 150 kVA which works out to be 120 kWe power It operates with a compression ratio of 14.5:1 Table presents the details of the engine configuration Method and experiments 3.1 Gas analysis, ultimate and proximate analysis The mass of the feedstock was determined using a measuring balance/scale before feeding into the gasifier hopper The ultimate and proximate analyses were done to determine the physical and chemical properties of the pine wood used for this study The calorific value of the material was determined using a CAL2K oxygen Bomb calorimeter Gas analysis was undertaken using a custom-built Gas and Temperature Profiling System (GTPS), which employs non-dispersive infrared gas sensors for measurement of methane, carbon monoxide, carbon dioxide and a palladium/nickel (Pd/Ni) gas sensor for measurement of hydrogen The differential voltage outputs are logged into a CR1000 data logger which comprises of a central processing unit (CPU), analog and digital inputs and outputs It has eight differential or 16 single-ended analog inputs for measuring voltages up to V The logged data are then downloaded into the computer and transformed to percentage composition [12] The calorific value of the gas (CVgas ) was determined from the percentage composition of combustible gases as shown in Equation CVgas ¼ ðCOvol *COHV Þ þ ðH2vol *H2HV Þ þ ðCH4vol *CH4HV Þ 100 (6) Table Electrical generator configuration Component Detail/Units Power output rating Compression ratio Nominal bore Stroke Cubic capacity Number of cylinder Dry weight Type of coolant Rated speed 120 kW 14.5:1 135 mm 152 mm 2611 L 12 2120 kg 50% ethylene glycol and 50% water 1500 RPM The electrical output of the generator was measured when powering a load bank connected to it The load bank was constructed using 12 kW water heating elements connected such that they draw maximum power from each of the three phases Fig shows an electrical circuit of the load bank designed using a personal computer simulation program with integrated emphasis (pspice) Pspice simulates the behavior of electrical circuit, hence allowing the evaluation of circuits without physically building the circuit This in turn saves money and time for the designer The electrical circuit in Fig represents the actual configuration of the load bank connected to draw power from the three phase generator The power generated from the constant 400 V line to line voltages arranged in star connection was at a desired frequency range of 50 Hze55 Hz It can be deduced from the circuit that each line contained four 12 kW heating elements connected in series (total power demanded by loads on each line is 48 kW) The line current under the full load condition was ideally 120 A and the total power dissipated by the elements from the three lines was 144 kW The total weight of material (Min) that entered the downdraft gasifier was estimated as follows: Min ¼ Ww þ Aw (7) Where Ww is the weight of wood in kg that was consumed in the gasifier and Aw is the mass (kg) of air used The air flow rate was measured in Nm3 using an anemometer and was latter converted to kg The total weight of output product Pout was also estimated as follows: Pout ¼ Gq þ Ash þ Fp (8) Where Gq and Fp are the total quantity of gas in kg and fine particles generated in kg respectively The total quantity of gas was determined from the gas production rate of the gasifier, which is 300 Nm3/hr for Johansson biomass gasifier Fine particles were measured using a measuring balance The energy balance of the downdraft gasifier was determined from total quantity of energy that went into the gasifier and the total quantity that came out as follows: Ein ¼ CVfuel  Ww (9) Eout ¼ CVgas  Gq (10) Where Ein and Eout is the total input energy and output energy in MJ CVfuel is the calorific value of fuel in MJ/kg and CVgas is the calorific value of producer gas in MJ/Nm3 698 N Nwokolo et al / Renewable Energy 71 (2014) 695e700 Fig Schematic of the load bank circuit drawn using Pspice The wood to producer gas conversion efficiency of the biomass downdraft gasifier was estimated according to Eq (11) CGE ¼ CVgas  Ww  100 CVfuel  Gq (11) The gas to power efficiency of the system was determined through Eq (12) as follows: GPE ¼ Elenergy  100 Eout (12) Where GPE represents gas to power efficiency and Elenergy is the total electrical energy produced from the generator Wood to electrical power production efficiency known as the overall efficiency of the system is given as WPE ¼ Elenergy  100 Ein (13) calorific value of wood This was used to determine the conversion efficiency of the system 4.2 Gas analysis Fig shows the gas profiles as obtained from the gas and temperature profiling system On average the percentage compositions of the gases are 29.6% of H2, 18.4% of CO, 18.57% of CO2 and 2.6% of CH4 Nitrogen makes up for the remaining composition of the gas, which is relatively high The high percentage of nitrogen is because the Johansson biomass gasifier is an air blown gasifier The calorific value of the producer gas was calculated using Eq (6) This resulted in an average value of 6.3 MJ/Nm3, which is within the range (4e7 MJ/Nm3) reported for air gasification [13,7] This is attributed to the higher quantity of the combustible gases obtained The use of air introduces a high quantity of nitrogen to the gas which explains the reason for low calorific value of the producer gas 4.3 Electrical performance Results and discussions 4.1 Wood analysis Table presents the proximate and ultimate analyses of a random sample of wood generated from the sawmill industry The proximate and ultimate analysis were determined to establish the suitability of the feedstock for gasification purposes The obtained moisture content is within the range required for downdraft gasifier Usually high moisture content is unfavorable during gasification since it lowers combustion zone temperature and thus leads to production of low quality gas High volatile matter content shows the ease with which the wood can be ignited The fixed carbon represents the carbon content of the wood which does not decompose easily at low temperature Low ash content minimizes the likelihood of slag formation at high temperature during the gasification process No sulfur was detected in the wood sample and oxygen was determined by difference The calorific value of the wood was found to be 16.34 MJ/kg which is within the known Table Proximate and ultimate analysis of wood Pine wood Proximate analysis % by weight MC VM 14 67.72 Ultimate analysis% by weight C H 47.51 6.524 FC 17.88 AC 0.4 N 0.095 O 45.87 The electrical output of the generator operated on a 100% producer gas was monitored using an energy meter During the operation of the generator the frequency varied between 50 Hz and 55 Hz The current in the three phase when the engine was operated at full load (when the electrical demand from the engine is equal to the electrical output deliverable by the engine) varied between 104 A and 114 A The variation in current occurred in the red and yellow phase while the blue phase remained constant at 107 A The voltage was fairly constant all through the operation of the engine for 194 mins An average power of 121.93 kW was obtained which is 1.93 kW above the power rating of the engine This shows that the gas fed into the engine was able to drive the engine to its power rating 4.4 Mass balance of the system Fig shows the mass balance analysis carried out to account for the materials that entered the gasifier and the products that came out In total 718.64 kg of air was fed into the gasifier along side with 424 kg of wood This worked out to be 63% of air and 37% of wood by mass fraction Translating further showed that every kg of wood was gasified by 1.69 kg of air This therefore corresponds to an equivalence ratio of 0.29 bearing in mind that on average 5.74 kg of air is required for complete combustion of kg of wood This lower equivalence ratio of 0.29 resulted in a gas heating value of 6.3 MJ/ Nm3 mentioned earlier which is higher when compared to 4.6 MJ/ Nm3 obtained at an equivalence ratio of 0.4 by Ref [7] This indicates that higher gas heating values are usually obtained at lower equivalence ratio Such is also the case with [14] findings where an N Nwokolo et al / Renewable Energy 71 (2014) 695e700 699 35 CO H2 CH4 CO2 27 32 30 Gas Volume (%) 25 20 15 10 12 17 22 Time (Minutes) 37 42 Fig Percentage compositions of producer gas increase in gasification air ratio from 0.16 to 0.26 resulted in an increase in gas heating value from 3.6 to 4.2 MJ/Nm3 Equivalence ratio is an important gasification parameter that should be carefully monitored The output product comprises of producer gas, ash and fine particles Out of 1142.64 kg of input material 89.77% was converted to gas and the remaining percentage to ash and fine particles Simplifying further showed that kg of wood produced 2.29 Nm3 of gas while consuming 1.69 kg of air The obtained gas production rate (GPR) of 2.29 Nm3 is slightly lower than the average value of 2.5 Nm3 reported [15] Table presents the summary of the input material and output product for the mass balance analysis 4.5 Energy balance and efficiency determination of the system The total energy input to the gasifier was estimated from the total kilogram of wood (424 kg) consumed and the calorific value of the wood (16.34 MJ/kg) This resulted in a total energy of 6928.16 MJ or 1924.49 kWh while the total energy output from the gasifier was estimated from total quantity of gas (969 Nm3) and heating value of the gas (6.3 MJ/Nm3) This gave a total energy of 6104.7 MJ or 1695.75 kWh This therefore indicates that 88.11% of the energy contained in the wood was converted to gas energy Hence energy lost while converting wood to gas was then determined by difference and amounted to 11.9% This loss is accounted for by the heat lost during the process of cleaning the gas and through the gasifier itself Downdraft Gasifier Producer gas (1025.76 kg) Wood (424 kg) + Air (718.64 kg) The conversion efficiency of the system was evaluated in three stages: First stage from wood to producer gas which was determined according to Eq (11) This resulted in an efficiency of 88.11% generally referred as cold gas efficiency or gasification efficiency Cold gas efficiency depends on the calorific value of gas and the quantity of gas generated as seen in the Eq (11) This value is much higher when compared to about 60e70% reported for wood gasifiers [15] In the second stage, producer gas to electric power generation efficiency was evaluated based on the total electrical energy generated during the running of the system The 150 kVA generator coupled to the producer gas engine generated a total of 394.2 kWh of electrical energy from 969 Nm3 of gas supplied to it Working it out further then shows that 2.458 Nm3 of gas was required to generate kWh of electrical energy This therefore gives a producer gas to electric power efficiency of 23.2% Lastly the overall efficiency of the system was calculated from total fuel consumed in the gasifier and the electrical energy generated from the engine A total of 424 kg of wood consumed resulted in a total electrical energy of 394.2 kWh Hence, a specific fuel consumption rate of 1.075 kg/ kWh and overall efficiency of 20.5% was obtained This is approximately equal to the overall efficiency obtained for a dual fired downdraft gasifier system [16] but lower than that obtained in a two stage gasification system by 6% [14] The specific fuel consumption rate of 1.075 kg/kWh obtained compares very closely to reported values of 1.1 kg/kWh [16] and 1.21 kg/kWh [17] The comparison to other references showed that lesser kilogram of wood is required by the present downdraft gasifier to produce kWh of electrical energy This is an evidence of stable gasifier operating conditions, low ash turn over and low charcoal yield of the Johansson biomass gasifier system Table presents a summary of some performance parameters obtained and their comparison with literature data Table Summary of Mass balance analysis of the system Input Ash + Fine particles (116.88 kg) Fig Schematic diagram for Mass balance of the system Output Component Unit Mass Component (kg) fraction (%) Pine wood 424.00 37.00 Producer gas Air 718.64 63.00 Ash þ fine particles Total 1142.64 100.00 Total Unit Mass (kg) fraction (%) 1025.76 89.77 116.88 10.23 1142.64 100 700 N Nwokolo et al / Renewable Energy 71 (2014) 695e700 Table Comparing experimental data with literature Biomass material Optimum ER CVgas (MJ/ Nm3) GPR (Nm3/ kg) CGE (%) Reference Fuel wood Furniture waste Wood chips Hazel nutshell Wood þ charcoal Pine wood 0.29 0.205 0.21 0.276 0.388 0.29 5.30 6.34 3.90 5.15 5.62 6.3 2.78 1.62 2.93 2.73 1.08 2.29 89.70 56.87 66.00 80.91 33.72 88.11 [16] [18] [14] [8] [19] Present study ER ¼ Equivalence ratio Conclusion The performance of a Johansson biomass gasifier evaluated in this study showed that the system was producing power above its power rating when operated at full load The calorific value of the gas was estimated to be 6.3 MJ/Nm3 at an equivalence ratio of 0.29 This agreed well with literature as summarized in Table A cold gas efficiency of 88.11% obtained indicates stability in the gasifier operating condition This was evident also in the calorific value of the gas obtained The study also revealed that every kWh of electrical energy generated consumed about 2.458 Nm3 of gas Acknowledgment The authors would like to acknowledge South African Clean Energy Solutions limited R8000 for funding the construction of the load bank and the experimental part of the research We would also like to acknowledge the National Research Foundation R40000, Eskom R4.5million and Govan Mbeki Research and development centre R8000 at the University of Fort Hare for Funding References [1] Chopra S, Jain A A review of fixed bed gasification systems for biomass, agricultural engineering international: the CIGR Ejournal, Invited Overview 2007;IX(5) [2] McKendry P Energy production from biomass (part 2): conversion technologies Bioresour Technol 2002;83:47e54 [3] Shrivastava V, Jha AK, Wamankar AK, Murugan S Performance and emission studies of a CI engine coupled with gasifier running in dual fuel mode Procedia Engineering 2013;51:600e8 [4] Sharma KA Experimental study on 75 kWth downdraft (biomass) gasifier system Renew Energy 2009;34:1726e33 [5] Damartzis T, Zabaniotou A Thermo chemical conversion of biomass to second generation biofuels through integrated process designd a review Renew Sustain Energy Rev 2011;15:366e78 [6] Warnecke R Gasification of biomass: comparison of fixed bed and fluidized bed gasifier Biomass Bioenergy 2000;18:489e97 [7] Martınez DJ, Lora Electo Eduardo Silva, Andrade VR, Jaen LR Experimental study on biomass gasification in a double air stage downdraft reactor Biomass Bioenergy 2011;35:3465e80 [8] Dogru M, Howrath CR, Akay G, Keskinler B, Malik AA Gasification of hazelnut shells in a downdraft gasifier Energy 2002;27:415e27 [9] Jayah HT, Aye L, Fuller JR, Stewart FD Computer simulation of a downdraft wood gasifier for tea drying Biomass Bioenergy 2003;25:459e69 [10] Barrio M, Fossum M, Hustad JE A small-scale stratified downdraft gasifier coupled to a gas engine for combined heat and power production Norwegian University of Science and Technology, Department of Thermal Energy and Hydro Power; 2007 7491 Trondheim, Norway Sintef Energy Research, Department of Thermal Energy, 7465 Trondheim, Norwary [11] Mamphweli NS, Meyer Edson Leroy Components and operation of the fixed bed downdraft system Johansson biomass gasifier Nova Science Publisher; 2012, ISBN 978-1-61209-681-0 [12] Mamphweli NS, Meyer EL Performance monitoring system for a biomass gasifier Journal of Engineering, Design and Technology 2013;11:7e18 [13] Holmgren MK, Berntsson T, Andersson E, Rydberg T System aspects of biomass gasification with methanol synthesis-Process concepts and energy analysis Energy 2012;45:817e28 [14] Wang Y, Yoshikawa K, Namioka T, Hashimoto Y Performance optimization of two-staged gasification system for woody biomass Fuel Process Technol 2007;88:243e50 [15] Rajvanshi AK Biomass gasification., published as chapter in book In: Goswami DY, editor Alternative Energy in Agriculture, Vol II CRC Press; 1986 pp 83e102 [16] Raman P, Ram KN, Gupta R A dual fired downdraft gasifier system to produce cleaner gas for power generation: design, development and performance analysis Energy 2013;54:302e14 [17] Sridhar G, Paul JP, Mukunda SH Biomass derived producer gas as a reciprocating engine fueldan experimental analysis Biomass Bioenergy 2001;21: 61e72 [18] Sheth NP, Babu VB Experimental studies on producer gas generation from wood wastein a downdraft biomass gasifier Bioresour Technol 2009;100: 3127e33 [19] Zainal ZA, Ali R, Quadir G, Seetharamu KN Experimental investigations of a downdraft biomass gasifier Biomass Bioenergy 2002;23:283e9 [20] Kumar A, Jones DD, Hanna AM Thermo chemical biomass gasification: a review of the current status of the technology Energies 2009;2:556e81