Design and performance analysis of a 500‐W heat source for radioisotope thermophotovoltaic converters

A radioisotope thermophotovoltaic (RTPV) system effectively converts the decay heat of radioisotopes into electricity via thermally radiated photons. In this work, a 500‐W thermal heat source unit including ²³⁸PuO₂ radioisotope fuel, shielding material, and selective emitter is designed from the viewpoint of radiation safety, thermal performance, and overall conversion efficiency by considering various shielding materials, fuel configurations, and packing factor (PF), defined as the ratio of fuel region volume to total heat source enclosure volume including fuel cladding and shield. The design study starts with a reference cubic configuration and extends to the more complicated configurations having separate cylindrical fuels. The results of the study showed that the heat source unit design suggested here can reduce the total radiation dose, peak neutron fluence, and maximum temperature using separate cylindrical fuel rods. For example, a design having a separated 3 × 3 cylindrical fuel rod array of 30% PF increases the overall efficiency by ~39% with similar maximum temperature and radiation doses in comparison with the reference heat source unit with a single cubic module and a 10% PF. This demonstrates the importance of the proper design of the RTPV heat source unit.     

Performance analysis of a latent heat storage system with phase change material for new designed solar collectors in greenhouse heating

The continuous increase in the level of greenhouse gas emissions and the rise in fuel prices are the main driving forces behind the efforts for more effectively utilize various sources of renewable energy. In many parts of the world, direct solar radiation is considered to be one of the most prospective sources of energy. In this study, the thermal performance of a phase change thermal storage unit is analyzed and discussed. The storage unit is a component of ten pieced solar air collectors heating system being developed for space heating of a greenhouse and charging of PCM. CaCl₂6H₂O was used as PCM in thermal energy storage with a melting temperature of 29 °C. Hot air delivered by ten pieced solar air collector is passed through the PCM to charge the storage unit. The stored heat is utilized to heat ambient air before being admitted to a greenhouse. This study is based on experimental results of the PCM employed to analyze the transient thermal behavior of the storage unit during the charge and discharge periods. The proposed size of collectors integrated PCM provided about 18–23% of total daily thermal energy requirements of the greenhouse for 3–4 h, in comparison with the conventional heating device.     

A gasoline fuel processor designed to study quick-start performance

A fast-start capability is a key requirement for on-board fuel processors for automotive fuel cell systems operating on gasoline fuel. This paper reports on the design and fabrication of a suitable fuel processor having this capability and discusses estimates of the start-up fuel consumption for the laboratory unit. Also discussed are the start-up strategy and the results of a start-up simulation, which showed that the fuel processor can deliver 90% of the rated hydrogen capacity in 60 s, producing a product gas that contains >30% hydrogen and <50 ppm carbon monoxide. The start-up fuel consumption was estimated on the basis of the thermal mass of the fabricated components; the 10-kW_e laboratory unit was estimated to require >2.9 MJ of fuel energy.     

Maximising the energy output of a PVT air system

Simultaneously generating both electricity and low grade heat, photovoltaic thermal (PVT) systems maximise the solar energy extracted per unit of collector area and have the added benefit of increasing the photovoltaic (PV) electrical output by reducing the PV operating temperature. A graphical representation of the temperature rise and rate of heat output as a function of the number of transfer units NTUs illustrates the influence of fundamental parameter values on the thermal performance of the PVT collector. With the aim of maximising the electrical and thermal energy outputs, a whole of system approach was used to design an experimental, unglazed, single pass, open loop PVT air system in Sydney. The PVT collector is oriented towards the north with a tilt angle of 34°, and used six 110 Wp frameless PV modules. A unique result was achieved whereby the additional electrical PV output was in excess of the fan energy requirement for air mass flow rates in the range of 0.03–0.05 kg/s m². This was made possible through energy efficient hydraulic design using large ducts to minimise the pressure loss and selection of a fan that produces high air mass flow rates (0.02–0.1 kg/s m²) at a low input power (4–85 W). The experimental PVT air system demonstrated increasing thermal and electrical PV efficiencies with increasing air mass flow rate, with thermal efficiencies in the range of 28–55% and electrical PV efficiencies between 10.6% and 12.2% at midday.     

Catalytic combustion of 1-butanol coupled with heat harvesting for compact power

A combustor paired with a heat-harvesting device, such as a thermoelectric or thermal photovoltaic device, can utilize high energy-dense liquid fuels while avoiding direct chemical-to-electrical conversion issues such as electrode and electrolyte poisoning. Therefore, the system is an attractive alternative to batteries and fuel cells for portable power applications. In the current study, a 1-butanol fed catalytic combustor using a Rh/Al₂O₃ catalyst was tested with a heat extractor, in this case being a stainless steel rod with a copper heat sink that was designed to thermally mimic a small thermoelectric module. The effects of residence time, fuel flow rate, and rod size on reactor/extractor temperatures and the energy balance were observed. Fuel-lean equivalence ratios were also studied and shown to have little effect on performance. Residence time does not have a direct effect; however, it does provide a catalytic stability limit for the fuel flow rate. The difference in the hot and cold side temperatures of the rod is dependent on the fuel flow rate and length of the rod. The greatest difference observed in these temperatures was 513 °C using the long-sized (15 cm) rod. The percentage of fuel energy conducted through the rod is only dependent on the rod size, with a maximum around 40% using the short rod. These results provide important design guidelines for the catalytic combustion of energy-dense liquid fuels as an excellent alternative heat source for either direct use or electrical power conversion.     

Investigation of nickel‐63 radioisotope‐powered GaN betavoltaic nuclear battery

This work describes the theoretical and experimental investigation of an in‐house produced ⁶³Ni radioisotope‐powered GaN‐based direct conversion (betavoltaic) nuclear battery. GaN p‐n junction device with 1‐mm² area was fabricated and irradiated by the ⁶³Ni plate source. Short‐circuit current and open‐circuit voltage of the battery were measured, and current‐voltage curves were plotted. The energy stored in battery, maximum power, and efficiency parameters were calculated. Monte Carlo modelling was used to investigate radioisotope's self‐absorption effect, the optimization of semiconductor and source thickness, transport, and penetration of beta particles in semiconductor junction. A large fraction of beta particle energy emitted from ⁶³Ni source is absorbed within 1 μm of the semiconductor junction on the basis of the simulation results. Epitaxial growth of GaN was performed using metal‐organic chemical vapour deposition (MOCVD) system. Monte Carlo simulation with MCNPX was used to determine optimum ⁶³Ni radioactive film thickness. ⁶³Ni film was electroplated on one face of 1‐mm² copper plate and mounted 1 mm over the semiconductor device. A ⁶³Ni source with an apparent activity of 0.31 mCi produced 0.1 ± 0.001 nA short‐circuit current (I_sc), 0.65 V ± 0.0022 open‐circuit voltage (V_oc), and 0.016 nW ± 0.0002 maximum power (P_max) in the semiconductor device. The filling factor (FF) of the betavoltaic cell was 25%, and the conversion efficiency (?) was 0.05%. Finally, experimental results were compared with theoretical calculations.     

Consequences of size increases and thermodynamic constraints on steam-powerplant availability: Comparison between nuclear and fossil-fueled units

By comparing the two largest thermoelectric units recently brought into operation in Italy (i.e., a 660-MW_e fossil-fueled supercritical unit and an 860-MW_e nuclear direct cycle BWR), we obtain information on the effects of size limits on powerplant availability. We assume that torsional vibrations are among the most severe causes of damage, while short circuits at the generator terminals cause the most serious fault in a power station. Computer-aided simulation of the dynamical behaviour of the two units shows the nuclear unit to be less severely stressed, thus producing longer life and better long-term reliability.     

A fuel cell powered unmanned aerial vehicle for low altitude surveillance missions

Boeing Research & Technology Europe has designed, developed and subsequently bench and flight tested, in a wide range of different operative conditions, an electric Unmanned Air Vehicle (UAV) powered by a hybrid energy source. The energy source features a 200 W_e Polymer Electrolyte Membrane (PEM) fuel cell system fed by a chemical hydride hydrogen generator that produces highly pure hydrogen at the fuel cell operating pressure from the controlled hydrolysis of Sodium Borohydride (NaBH₄), resulting in 900 Wh of energy from 1 L of chemical solution. Equipped also with high specific energy Lithium Polymer batteries, this fuel cell powered UAV is able to achieve flight durations close to 4 h.This paper summarizes the aircraft and systems design, the results of the bench and flight tests along with the main challenges faced during this development and the lessons learned for future optimization.     

Techno-economic and environmental performances of glycerol reforming for hydrogen and power production with low carbon dioxide emissions

This paper is evaluating from the conceptual design, thermal integration, techno-economic and environmental performances points of view the hydrogen and power generation using glycerol (as a biodiesel by-product) reforming processes at industrial scale with and without carbon capture. The evaluated hydrogen plant concepts produced 100,000 Nm³/h hydrogen (equivalent to 300 MW_th) with negligible net power output for export. The power plant concepts generated about 500 MW net power output. Hydrogen and power co-generation was also assessed. The CO₂ capture concepts used alkanolamine-based gas–liquid absorption. The CO₂ capture rate of the carbon capture unit is at least 90%, the carbon capture rate of the overall reforming process being at least 70%. Similar designs without carbon capture have been developed to quantify the energy and cost penalties for carbon capture. The various glycerol reforming cases were modelled and simulated to produce the mass & energy balances for quantification of key plant performance indicators (e.g. fuel consumption, energy efficiency, ancillary energy consumption, specific CO₂ emissions, capital and operational costs, production costs, cash flow analysis etc.). The evaluations show that glycerol reforming is promising concept for high energy efficiency processes with low CO₂ emissions.     

Experimental investigations on start-up performance of static nuclear reactor thermal prototype

Nuclear power is most suited to satisfy the energy demands of future deep space exploration. In this paper, we propose a static nuclear reactor (the nuclear static thermoelectric reactor [NUSTER]), which offers the advantages of superior modularization, simplification, a fully static state, and passive operation. Based on the conceptual design of a static nuclear reactor, an electrical heating principle prototype was designed and fabricated to validate the feasibility of the fully static passive energy conversion concept. Skutterudite thermoelectric generators (TEGs) were used for static energy conversion, and potassium heat pipes were employed for passive heat transfer. The system start-up performance, restart performance, and thermoelectric performance were investigated using the thermal principle prototype. We proposed a new approach to analyze the heat pipe start-up process based on the heat transfer performance. The experimental results indicated that the restart process can be used to reduce the start-up time, because the low heat flux stage is avoided. During the start-up process, the TEGs hot side heat flux and temperature difference were gradually established, and the TEGs open circuit voltage and power density gradually increased. A maximum open circuit voltage and power density of 38.2 V and 0.92 W/cm², respectively, were achieved when the TEGs temperature difference reached 575°C. The high performance of the thermal principle prototype demonstrated the feasibility of the NUSTER conceptual design, and the experimental data can serve as a valuable reference for optimization of static reactor designs.     

Conceptual design and selection of a biodiesel fuel processor for a vehicle fuel cell auxiliary power unit

Within the European project BIOFEAT (biodiesel fuel processor for a fuel cell auxiliary power unit for a vehicle), a complete modular 10 kW_e biodiesel fuel processor capable of feeding a PEMFC will be developed, built and tested to generate electricity for a vehicle auxiliary power unit (APU). Tail pipe emissions reduction, increased use of renewable fuels, increase of hydrogen-fuel economy and efficient supply of present and future APU for road vehicles are the main project goals. Biodiesel is the chosen feedstock because it is a completely natural and thus renewable fuel.Three fuel processing options were taken into account at a conceptual design level and compared for hydrogen production: (i) autothermal reformer (ATR) with high and low temperature shift (HTS/LTS) reactors; (ii) autothermal reformer (ATR) with a single medium temperature shift (MTS) reactor; (iii) thermal cracker (TC) with high and low temperature shift (HTS/LTS) reactors. Based on a number of simulations (with the AspenPlus^® software), the best operating conditions were determined (steam-to-carbon and O₂/C ratios, operating temperatures and pressures) for each process alternative. The selection of the preferential fuel processing option was consequently carried out, based on a number of criteria (efficiency, complexity, compactness, safety, controllability, emissions, etc.); the ATR with both HTS and LTS reactors shows the most promising results, with a net electrical efficiency of 29% (LHV).     

Renewable and hydrogen energy integrated house

The residential sector accounts for about a third of the total world energy consumption. Energy efficiency, Renewable Energy Sources and Hydrogen can play an important role in reducing the consumptions and the emissions and improving the energy security if integrated (Efficiency, Res, Hydrogen) systems are developed and experimented. The paper analyzes a real residential 100 square meters house, where energy efficiency measures and RES technologies have been applied, sizing a hydrogen system (electrolyzer, metal hydrides and fuel cell) for power backup, taking into consideration its dynamic behavior, experimentally determined. The technologies used are already available in the market and, except hydrogen technologies, sufficiently mature. Through energy efficiency technologies (insulation, absorbers, etc), the maximum electrical and thermal power needed decreases from 4.4 kW_e to 1.7 kW_e (annual consumption from 5000 kWh to 1200 kWh) and from 5.2 kW_t to 1.6 kW_t (annual consumption from 14,600 kWh to 4500 kWh) respectively. With these reduced values it has been possible to supply the consumptions entirely by small photovoltaic and solar thermal plants (less than 10 m² each). The hydrogen backup even if remains the most expensive (versus traditional batteries and gasoline generator), satisfying all the electric needs for one day, increases the security and allows net metering. Moreover the low-pressure hydrogen storage system through metal hydrides guarantees system safety too. Finally the system modularity can also satisfy higher energy production.     

Segmented thermoelectric generator: exponential area variation in leg

The innovative design of segmented thermoelectric generator with exponential area variation is introduced. Thermal efficiency and power output are assessed for various values of the design parameter (a = (L/x) ln[A_a/A(x)], where A_a is constant, and a is the dimensionless geometric parameter, L is the pin length, and A(x) is the pin cross‐sectional area), external load parameter (R_L/R₀, ratio of external electrical resistance to reference electrical resistance), and temperature parameter (θ = T_low/T_high, ratio of cold junction temperature to high junction temperature). The device efficiency obtained is validated through the previous experimental data for various hot and cold junction temperature differences. The findings reveal that thermal efficiency resulted from the current study agrees well with the experimental data. The innovative design of the segmented thermoelectric generator with exponentially decaying pin configuration enhances the thermal efficiency and output power as compared with the device having a single material pin configuration. Increasing temperature ratio results in the reduction in the thermal efficiency and the output power of thermoelectric generator. In addition, lowering the external load parameter improves the thermal efficiency and the output power of the thermoelectric device. The design parameter that maximizes the thermal efficiency of the thermoelectric generator does not maximize the device output power.     

Performance of hydrogen and power co-generation system based on chemical looping hydrogen generation of coal

An integrated hydrogen and power co-generation system based on slurry-feed coal gasification and chemical looping hydrogen generation (CLH) was proposed with Shenhua coal as fuel and Fe₂O₃/MgAl₂O₄ as an oxygen carrier. The sensitivity analyses of the main units of the system were carried out respectively to optimize the parameters. The syngas can be converted completely in the fuel reactor, and both of the fuel reactor and steam reactor can maintain heat balance. The purity of hydrogen produced after water condensation is 100%. The energy and exergy analyses of the proposed system were studied. Pinch technology was adopted to get a reasonable design of the heat transfer network, and it is found pinch point appears at the hot side temperature of 224.7 °C. At the given status of the proposed system, the hydrogen yield is 1040.11 kg·h⁻¹ and the CO₂ capture rate is 94.56%. At the same time, its energy and exergy efficiencies are 46.21% and 47.22%, respectively. According to exergy analysis, the degree of exergy destruction is ranked. The gasifier unit has the most serious exergy destruction, followed by chemical looping hydrogen generation unit and the heat recovery steam generator unit.     

The perspectives of energy production from coal‐fired power plants in an enlarged EU

The aim of this paper is to present the current status of the coal‐fired power sector in an enlarged EU (EU‐15 plus EU member candidate states) in relation with the main topics of the European Strategy for the energy production and supply. It is estimated that 731 thermoelectric units, larger than 100 MW_e, are operating nowadays, and their total installed capacity equals to 200.7 GW_e. Coal contribution to the total electricity generation with reference to other fuel sources, is by far more intensive in the non‐EU part (EU member candidate states), compared to the EU member states. It is expected that even after the enlargement, the European Union will strongly being related to coal. Enlargement will bring additional factors into play in order to meet the requirements of rising consumption, growing demand for conventional fuels and increasing dependence on imports. Besides the technology, boiler size, efficiency, age and environmental performance will determine the necessities of the coal‐fired power sector in each country. Depending on the case, lifetime extension measures in operating coal‐fired power plants or clean coal technologies can play an important role towards the energy sector restructuring. Low efficiency values in the non‐EU coal‐fired units and heavily aged power plants in EU countries will certainly affect decisions in favour of upgrading or reconstruction. The overall increase of efficiency, the reduction of harmful emissions from generating processes and the co‐combustion of coal with biomass and wastes for generating purposes indicate that coal can be cleaner and more efficient. Additionally, plenty of rehabilitation projects based on CCT applications, have already been carried out or are under progress in the EU energy sector. The proclamations of the countries' energy policies in the coming decades, includes integrated renovation concepts of the coal‐fired power sector. Further to the natural gas penetration in the electricity generation and CO₂ sequestration and underground storage, the implementation of CCT projects will strongly contribute to the reduction of CO₂ emissions in the European Union, according to the targets set in the Kyoto protocol. In consequence, clean coal technologies can open up new markets not only in the EU member candidate states, but also in other parts of the world. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal‐hydraulic design features of a micronuclear reactor power source applied for multipurpose

Microheat pipe cooled reactor power source (HRP) designed for space or underwater vehicles meets the future demands, such as safer structure, longer operating time, and fewer mechanical moving parts. In this paper, potassium heat pipe cooled reactor power source system which generates 50 kWe electricity is proposed. The reactor core using uranium nitride fuel is cooled by 37 potassium high‐temperature heat pipes. The shields are designed as tungsten and water, and reactor reactivity is controlled by control drums. The thermoelectric generator (TEG) consists of thermoelectric conversion units and seawater cooler. The thermoelectric conversion units convert thermal energy to electric energy through the high‐performance thermoelectric material. A code applied for designing and analyzing the reactor power system is developed. It consists of multichannel reactor core model, heat pipe model using thermal resistance network, thermoelectric conversion, and thermal conductivity model. Then, the sensitivity analysis is performed on two key parameters including the length of the heat pipe condensation section and the cold junction temperature of the TE cell. Meanwhile, the steady‐state calculations are conducted. Results show that the maximum fuel temperature is 938 K located in the center of reactor core and the outlet temperature of coolant reaches 316 K. Both of them are within the limitation. It is concluded that the preliminary design of HPR design is reasonable and reliable. The designed residual heat removal system has sufficient safety margin to release the decay heat of the reactor. This research provides valuable analysis for the application of micronuclear power source.     

A new approach to empirical electrical modelling of a fuel cell,an electrolyser or a regenerative fuel cell

In terms of fuel cell steady-state performance modelling, many electrical models have been developed either from a theoretical point of view or from an empirical point of view. The model described in this article is from the empirical point of view approach. This model enables to simulate both fuel cells and electrolysers V–J curves (cell voltage versus current density) in typical conditions. This model is particularly adapted to regenerative fuel cell (RFC) simulation. It is a four degree-of-freedom model and it is convergent near zero current. It depends on the stack temperature and the oxygen partial pressure. The regions where mass transfer limitations occur have not been modelled, because they are usually avoided for efficiency or thermal reasons. The parameters have been fitted with a 4 kW_e proton exchange membrane fuel cell (PEMFC) and a 3.6 kW_e electrolyser. The electrical equations and the experimental data are well correlated.     

Allotrope carbon materials in thermal interface materials and fuel cell applications: A review

The performance of allotrope carbon materials has been explored because of their superior properties in energy system applications. This review provides an understanding of the current work focusing on the applications of selected carbon materials in important energy systems, focus on thermal interface materials (TIMs), and fuel cell applications. This article begins with the introduction of TIMs and fuel cell in general working principle and presents details on carbon materials. The discussion focuses on updates from the latest research work and addresses current challenges and opportunities for research toward TIMs and fuel cell applications. The optimum performance of TIMs was seen when thermal conductivity achieved at a maximum of 3000 W (m K)⁻¹ by using vertically aligned carbon nanotubes (CNTs) and a minimum internal thermal resistance of 0.3 mm² K W⁻¹. Meanwhile for fuel cell, the platinum/CNTs catalyst applied proton exchange membrane fuel cell achieved high power density of 661 mW cm⁻² in the presence of Nafion electrolyte membrane. This review provides insights for scientists about the use of carbon materials, especially in energy system applications.     

Experimental performance evaluation of an ammonia-fuelled microchannel reformer for hydrogen generation

Microchannel reactors appear attractive as integral parts of fuel processors to generate hydrogen (H₂) for portable and distributed fuel cell applications. The work described in this paper evaluates, characterizes, and demonstrates miniaturized H₂ production in a stand-alone ammonia-fuelled microchannel reformer. The performance of the microchannel reformer is investigated as a function of reaction temperature (450–700 °C) and gas-hourly-space-velocity (6520–32,600 Nml g_cat⁻¹ h⁻¹). The reformer operated in a daily start-up and shut-down (DSS)-like mode for a total 750 h comprising of 125 cycles, all to mimic frequent intermittent operation envisaged for fuel cell systems. The reformer exhibited remarkable operation demonstrating 98.7% NH₃ conversion at 32,600 Nml g_cat⁻¹ h⁻¹ and 700 °C to generate an estimated fuel cell power output of 5.7 W_e and power density of 16 kW_e L⁻¹ (based on effective reactor volume). At the same time, reformer operation yielded low pressure drop (<10 Pa mm⁻¹) for all conditions considered. Overall, the microchannel reformer performed sufficiently exceptional to warrant serious consideration in supplying H₂ to fuel cell systems.