An experimental investigation of a natural circulation heat pipe system applied to a parabolic trough solar collector steam generation system

A U-type natural circulation heat pipe system is designed and applied to a parabolic trough solar collector for generating mid-temperature steam. Thermal performance of the heat pipe system is investigated experimentally. A detailed heat transfer analysis is performed on thermal behaviors of the system, especially the solar collector. The results show that the system can generate mid-temperature steam of a pressure up to 0.75 MPa. The thermal efficiency is found to be 38.52% at discharging pressure of 0.5 MPa during summer time.     

Performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) using CO2 as heat transfer fluid

In this paper, a performance and cost assessment of Integrated Solar Combined Cycle Systems (ISCCSs) based on parabolic troughs using CO₂ as heat transfer fluid is reported on. The use of CO₂ instead of the more conventional thermal oil as heat transfer fluid allows an increase in the temperature of the heat transfer fluid and thus in solar energy conversion efficiency. In particular, the ISCCS plant considered here was developed on the basis of a triple-pressure, reheated combined cycle power plant rated about 250 MW. Two different solutions for the solar steam generator are considered and compared.The results of the performance assessment show that the solar energy conversion efficiency ranges from 23% to 25% for a CO₂ maximum temperature of 550 °C. For a CO₂ temperature of 450 °C, solar efficiency decreases by about 1.5–2.0% points. The use of a solar steam generator including only the evaporation section instead of the preheating, evaporation and superheating sections allows the achievement of slightly better conversion efficiencies. However, the adoption of this solution leads to a maximum value of the solar share of around 10% on the ISCCS power output. The solar conversion efficiencies of the ISCCS systems considered here are slightly greater than those of the more conventional Concentrating Solar Power (CSP) systems based on steam cycles (20–23%) and are very similar to the predicted conversion efficiencies of the more advanced direct steam generation solar plants (22–27%).The results of a preliminary cost analysis show that due to the installation of the solar field, the electrical energy production cost for ISCCS power plants increases in comparison to the natural gas combined cycle (NGCC). In particular, the specific cost of electrical energy produced from solar energy is much greater (about two-fold) than that of electrical energy produced from natural gas.     

Steam condensing – liquid CO2 boiling heat transfer in a steam condenser for a new heat recovery system

In a new waste heat recovery system, waste heat is recovered from steam condensers through cooling by liquid CO₂ instead of seawater, taking advantage of effective boiling heat transfer performance; the heat is subsequently used for local heat supply. The steam condensing – liquid CO₂ boiling heat transfer performance in a steam condenser with a shell and a helical coil non-fin tube was studied both numerically and experimentally. A heat transfer numerical model was constructed from two models developed for steam condensation and for liquid CO₂ boiling. Experiments were performed to verify the model at a steam pressure range of 3.2–5 kPa and a CO₂ saturation pressure range of 5–6 MPa. Overall heat transfer coefficients obtained from the numerical model agree with the experimental data within ±5%. The numerical estimations show that the boiling local heat transfer coefficient reaches a maximum value of 26 kW/m² K. This value is almost one order higher than that of a conventional water-cooled condenser.     

Numerical analysis and optimization of a spectrum splitting concentration photovoltaic–thermoelectric hybrid system

This paper presents the numerical modeling and optimization of a spectrum splitting photovoltaic–thermoelectric (PV–TE) hybrid system. In this work, a simulation model is established in consideration of solar concentration levels and several heat dissipation rates. Exemplarily, the performance of a hybrid system composed of a GaAs solar cell and a skutterudites CoSb₃ solar thermoelectric generator (TEG) is simulated. Analysis under different conditions has been carried out to evaluate the electrical and thermal performance of the hybrid system. Results show that the cutoff-wavelength of the GaAs–CoSb₃ hybrid system is mainly determined by the band gap of solar cell, when the solar concentration ratio is ranged between 550 to 770 and heat transfer coefficient h = 3000–4500 W/m² K, the hybrid system has good electrical performance and low operating temperatures. Based on the analysis of the GaAs–CoSb₃ hybrid system, guidelines for the PV–TE system design are proposed. It is also compared with a PV-only system working under the same cooling condition; results show that the PV–TE hybrid system is more suitable for working under high concentrations.     

Consequences of a carbon tax on household electricity use and cost,carbon emissions,and economics of household solar and wind

The study was conducted to determine the consequences of a carbon tax, equal to an estimated social cost of carbon of $37.2/Mg, on household electricity cost, and to determine if a carbon tax would be sufficient to incentivize households to install either a grid-tied solar or wind system. U.S. Department of Energy hourly residential profiles for five locations, 20 years of hourly weather data, prevailing electricity pricing rate schedules, and purchase prices and solar panel and wind turbine power output response functions, were used to address the objectives. Two commercially available household solar panels (4 kW, 12 kW), two wind turbines (6 kW, 12 kW), and two price rate structures (traditional meter, smart meter) were considered. Averaged across the five households, the carbon tax is expected to reduce annual consumption by 4.4% (552 kWh/year) for traditional meter households and by 4.9% (611 kWh/year) for households charged smart meter rates. The carbon tax increases electricity cost by 19% ($202/year). For a household cost of $202/year the carbon tax is expected to reduce social costs by $11. Annual carbon tax collections of $234/household are expected. Adding the carbon tax was found to be insufficient to incentivize households to install either a solar panel or wind turbine system. Installation of a 4 kW solar system would increase the annual cost by $1546 (247%) and decrease CO₂ emissions by 38% (2526 kg) valued at $94/household. The consequence of a carbon tax would depend largely on how the proceeds of the tax are used.     

Reactivity study on oxygen carriers for solar-hybrid chemical-looping combustion of di-methyl ether

A power cycle with the solar-hybrid chemical-looping combustion can simultaneously provide a new insight for treating with the problem of large energy penalty for CO₂ capture in the energy system and achieve the efficient utilization of the mid-temperature solar thermal energy. Experiments were implemented on a thermo-gravimetrical reactor with di-methyl ether (DME) as fuel to identify suitable looping material for this system. Oxygen carriers, with Fe₂O₃, NiO, and CoO as solid reactants and Al₂O₃, MgAl₂O₄, and YSZ as binders, were prepared by dissolution method. Compared with Fe₂O₃ and NiO, CoO has higher reactivity in the research temperature range of 673–773 K, and the suitable reduction temperature of CoO with DME is around 723 K. Carbon deposition in the reduction process of DME with CoO/CoAl₂O₄ can be completely suppressed by adding water vapor to the gaseous reactant, and the optimal range of H₂O/DEM is around 1.5–2.0. Scanning electron microscopy was used to characterize the morphological features of the fresh and the cycling used oxygen carriers. For CoO/YSZ, big grains formed after cyclic reactions, but no apparent change was found for CoO/CoAl₂O₄. The findings of this paper provide a promising looping material candidate for the solar-hybrid chemical-looping combustion.     

Hydrogen production with integrated microchannel fuel processor for portable fuel cell systems

An integrated microchannel methanol processor was developed by assembling unit reactors, which were fabricated by stacking and bonding microchannel patterned stainless steel plates, including fuel vaporizer, heat exchanger, catalytic combustor and steam reformer. Commercially available Cu/ZnO/Al₂O₃ catalyst was coated inside the microchannel of the unit reactor for steam reforming. Pt/Al₂O₃ pellets prepared by ‘incipient wetness’ were filled in the cavity reactor for catalytic combustion. Those unit reactors were integrated to develop the fuel processor and operated at different reaction conditions to optimize the reactor performance, including methanol steam reformer and methanol catalytic combustor. The optimized fuel processor has the dimensions of 60 mm × 40 mm × 30 mm, and produced 450sccm reformed gas containing 73.3% H₂, 24.5% CO₂ and 2.2% CO at 230–260 °C which can produce power output of 59 Wt.     

Optimization of intercooling compression in CO2 capture systems

Compression of CO₂ is an essential process in the development of carbon capture and storage (CCS) technologies. In spite of power requirements for CO₂ compression could be as much as 100 kWe per tonne CO₂, the minimization of energy requirements has received little attention in the literature. Although intercooling compression reduces power requirements, it introduces important cooling necessities that could be minimized.The aim of this paper is the integration of intercooling compression into the low-pressure part of a steam cycle to take advantage of the intercooling heat and analyse the energetic and economical results under different assumptions. Simulation and optimization have been performed in order to evaluate the intercooling configuration, energy requirements and the most cost-effective integration. Results have shown reduction in compression power requirement around 40% and reduction of the incremental COE around 23%. Proposed integration could be used to increase the efficiency of CO₂ capture processes and, therefore, to reduce the CO₂ capture cost.     

Solar hybrid systems with thermoelectric generators

The possibility of using of thermoelectric generators in solar hybrid systems has been investigated. Four systems were examined, one working without radiation concentration, of the traditional PV/Thermal geometry, but with TEGs between the solar cells and heat extractor, and three other using concentrators, namely: concentrator – TEG ? heat extractor, concentrator ? PV cell ? TEG ? heat extractor, and PV cell – concentrator – TEG – heat extractor. The TEGs based on traditional semiconductor material Bi₂Te₃ and designed for temperature interval of 50–200 °C were studied experimentally. It was found that the TEG’s efficiency has almost linear dependence on the temperature difference ΔT between its plates, reaching 4% at ΔT = 155 °C (hot plate at 200 °C) with 3 W of power generated over the matched load. The temperature dependencies of current and voltage are also linear; accordingly, the power generated has quadratic temperature dependence. The experimental parameters, as well as parameters of two advanced TEGs taken from the literature, were used for estimation of performance of the hybrid systems. The conclusions are drawn in relation to the efficiency at different modes of operation and the cost of hybrid systems, as well as some recommendations in relation to optimal solar cells for applications in these systems.     

Maximum-power-point tracking control of solar heating system

The present study developed a maximum-power point tracking control (MPPT) technology for solar heating system to minimize the pumping power consumption at an optimal heat collection. The net solar energy gain Q_net (=Q_s ? W_p/η_e) was experimentally found to be the cost function for MPPT with maximum point. The feedback tracking control system was developed to track the optimal Q_net (denoted Q_max). A tracking filter which was derived from the thermal analytical model of the solar heating system was used to determine the instantaneous tracking target Q_max(t). The system transfer-function model of solar heating system was also derived experimentally using a step response test and used in the design of tracking feedback control system. The PI controller was designed for a tracking target Q_max(t) with a quadratic time function. The MPPT control system was implemented using a microprocessor-based controller and the test results show good tracking performance with small tracking errors. It is seen that the average mass flow rate for the specific test periods in five different days is between 18.1 and 22.9 kg/min with average pumping power between 77 and 140 W, which is greatly reduced as compared to the standard flow rate at 31 kg/min and pumping power 450 W which is based on the flow rate 0.02 kg/s m² defined in the ANSI/ASHRAE 93-1986 Standard and the total collector area 25.9 m². The average net solar heat collected Q_net is between 8.62 and 14.1 kW depending on weather condition. The MPPT control of solar heating system has been verified to be able to minimize the pumping energy consumption with optimal solar heat collection.     

Decarbonized hydrogen and electricity from natural gas

This paper discusses configuration, attainable performances and thermodynamic features of stand-alone plants for the co-production of de-carbonized hydrogen and electricity from natural gas (NG) based on commercially available technology.We focus on the two basic technologies currently used in large industrial applications: fired tubular reformer (FTR) and auto-thermal reformer (ATR). In both cases we assume that NG is pre-heated and humidified in a saturator providing water for the reforming reaction; this reduces the amount of steam to be bled from the power cycle and increases electricity production. Outputs flows are made available at conditions suitable for transport via pipeline: 60 bar for pure hydrogen, 150 bar for pure CO₂. To reduce hydrogen compression power requirements reforming is carried out at relatively high pressures: 25 bar for FTR, 70 bar for ATR. Reformed gas is cooled and then passed through two water–gas shift reactors to optimize heat recovery and maximize the conversion to hydrogen. In plants with CO₂ capture, shifted gas goes through an amine-based chemical absorption system that removes most of the CO₂. Pure hydrogen is obtained by pressure swing absorption (PSA), leaving a purge gas utilized to fire the reformer (in FTR) and to boost electricity production.For the power cycle we consider conventional steam cycles (SC) and combined cycles (CC). The scale of plants based on a CC is determined by the gas turbine. To maintain NG input within the same range (around 1200 MW), we considered a General Electric 7FA for ATR, a 6FA for FTR. The scale of plants with SC is set by assuming the same NG input of the corresponding CC plant.Heat and mass balances are evaluated by a model accounting for the constraints posed by commercial technology, as well as the effects of scale. Results show that, from a performance standpoint, the technologies of choice for the production of de-carbonized hydrogen from NG are FTR with SC or ATR with CC. When operated at high steam-to-carbon ratios, the latter reach CO₂ emissions chargeable to hydrogen of 10–11 kg of CO₂ per GJ_LHV—less than 20% of NG—with an equivalent efficiency of hydrogen production in excess of 77%.     

Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis

Large-scale atmospheric removal of greenhouse gases (GHGs) including methane, nitrous oxide and ozone-depleting halocarbons could reduce global warming more quickly than atmospheric removal of CO₂. Photocatalysis of methane oxidizes it to CO₂, effectively reducing its global warming potential (GWP) by at least 90%. Nitrous oxide can be reduced to nitrogen and oxygen by photocatalysis; meanwhile halocarbons can be mineralized by red-ox photocatalytic reactions to acid halides and CO₂. Photocatalysis avoids the need for capture and sequestration of these atmospheric components. Here review an unusual hybrid device combining photocatalysis with carbon-free electricity with no-intermittency based on the solar updraft chimney. Then we review experimental evidence regarding photocatalytic transformations of non-CO₂ GHGs. We propose to combine TiO₂-photocatalysis with solar chimney power plants (SCPPs) to cleanse the atmosphere of non-CO₂ GHGs. Worldwide installation of 50,000 SCPPs, each of capacity 200 MW, would generate a cumulative 34 PWh of renewable electricity by 2050, taking into account construction time. These SCPPs equipped with photocatalyst would process 1 atmospheric volume each 14–16 years, reducing or stopping the atmospheric growth rate of the non-CO₂ GHGs and progressively reducing their atmospheric concentrations. Removal of methane, as compared to other GHGs, has enhanced efficacy in reducing radiative forcing because it liberates more _°OH radicals to accelerate the cleaning of the troposphere. The overall reduction in non-CO₂ GHG concentration would help to limit global temperature rise. By physically linking greenhouse gas removal to renewable electricity generation, the hybrid concept would avoid the moral hazard associated with most other climate engineering proposals.     

Hybrid sulfur flowsheets using PEM electrolysis and a bayonet decomposition reactor

A conceptual design is presented for a hybrid sulfur process for the production of hydrogen using a high-temperature nuclear heat source to split water. The process combines proton exchange membrane-based SO₂-depolarized electrolyzer technology being developed at Savannah River National Laboratory with silicon carbide bayonet decomposition reactor technology being developed at Sandia National Laboratories. Both are part of the US DOE Nuclear Hydrogen Initiative. The flowsheet otherwise uses only proven chemical process components. Electrolyzer product is concentrated from 50 wt% sulfuric acid to 75 wt% via recuperative vacuum distillation. Pinch analysis is used to predict the high-temperature heat requirement for sulfuric acid decomposition. An Aspen Plus? model of the flowsheet indicates 340.3 kJ high-temperature heat, 75.5 kJ low-temperature heat, 1.31 kJ low-pressure steam, and 120.9 kJ electric power are consumed per mole of H₂ product, giving an LHV efficiency of 35.3% (41.7% HHV efficiency) if electric power is available at a conversion efficiency of 45%.     

Technical and economic assessment of grid-independent hybrid photovoltaic–diesel–battery power systems for commercial loads in desert environments

Solar photovoltaic (PV) hybrid system technology is a hot topic for R&D since it promises lot of challenges and opportunities for developed and developing countries. The Kingdom of Saudi Arabia (KSA) being endowed with fairly high degree of solar radiation is a potential candidate for deployment of PV systems for power generation. Literature indicates that commercial/residential buildings in KSA consume an estimated 10–45% of the total electric energy generated. In the present study, solar radiation data of Dhahran (East-Coast, KSA) have been analyzed to assess the techno-economic viability of utilizing hybrid PV–diesel–battery power systems to meet the load requirements of a typical commercial building (with annual electrical energy demand of 620,000 kW h). The monthly average daily solar global radiation ranges from 3.61 to 7.96 kW h/m². NREL's HOMER software has been used to carry out the techno-economic viability. The simulation results indicate that for a hybrid system comprising of 80 kWp PV system together with 175 kW diesel system and a battery storage of 3 h of autonomy (equivalent to 3 h of average load), the PV penetration is 26%. The cost of generating energy (COE, US$/kW h) from the above hybrid system has been found to be 0.149 $/kW h (assuming diesel fuel price of 0.1 $/L). The study exhibits that for a given hybrid configuration, the operational hours of diesel generators decrease with increase in PV capacity. The investigation also examines the effect of PV/battery penetration on COE, operational hours of diesel gensets for a given hybrid system. Emphasis has also been placed on unmet load, excess electricity generation, percentage fuel savings and reduction in carbon emissions (for different scenarios such as PV–diesel without storage, PV–diesel with storage, as compared to diesel-only situation), cost of PV–diesel–battery systems, COE of different hybrid systems, etc.     

Optical efficiency of a PV–thermal hybrid CPC module for high latitudes

The advantage of PV–thermal hybrid systems is their high total efficiency. By using concentrating hybrid systems, the cost per energy produced is reduced due to simultaneous heat and electricity production and a reduced PV cell area. In this article, the optical efficiency of a water-cooled PV–thermal hybrid system with low concentrating aluminium compound parabolic concentrators is discussed. The system was built in 1999 in Älvkarleby, Sweden (60.5° N, 17.4° E) with a geometric concentration ratio of C=4 and 0.5 kW_p electric power. The yearly output is 250 kWh of electricity per square metre solar cell area and 800 kWh of heat at low temperatures per square metre solar cell area. By using numerical data from optical measurements of the components (glazing, reflectors, and PV cells) the optical efficiency, η_opt, of the PV–CPC system has been determined to be 0.71, which is in agreement with the optical efficiency as determined from thermal and electrical measurements. Calculations show that optimised antireflection-treated glazing and reflectors could further increase the electric power yield.     

CO2 emission and avoidance in mobile applications

The present paper analyzes the CO₂ emissions from mobile communications and portable wireless electronic devices in the Korea environment. The quantitative and qualitative contributions to CO₂ emission reduction of the substitution of renewable energy for traditional electricity as the power supply in these devices are also investigated.Firstly, the national CO₂ emission coefficient is temporarily estimated as 0.504 tCO₂/MWh, which can be regarded as the basis for calculating CO₂ emissions in mobile devices. The total annual CO₂ emissions from mobile devices is calculated as approximately 1.4 million tons, comprising 0.3 million tCO₂ for portable wireless electronic devices and 1.1 million tCO₂ for electric equipment required for mobile communication service.If renewable energy sources are substituted for traditional electricity sources in the supply for mobile devices, solar cell and wind turbine systems can reduce CO₂ emissions by about 87% and 97%, respectively. However, the use of fuel cell systems will only slightly reduce the CO₂ emissions. However, the use of the direct methanol fuel cell system can release 8% more CO₂ emissions than that emitted by using traditional electricity sources.     

High-temperature carbonate/MgO composite materials as thermal storage media for double-walled solar reformer tubes

The composite materials of molten alkali-carbonate/MgO-ceramics are examined as thermal storage media in a tubular reformer using a double-walled reactor tube of a laboratory scale. The concept of a double-walled reformer tube is proposed as a solar tubular reformer and involves packing a molten salt/ceramic composite material in the annular region between the internal catalyst tube and the exterior solar absorber wall. The composite materials of Na₂CO₃, K₂CO₃, and Li₂CO₃ with magnesia are tested as thermal storage media. The reforming performances of the composite materials are tested in the cooling mode of the double-walled reactor tube. The experimental result obtained under feed gas mixture of CH₄/CO₂ = 1:3 at 1 atm shows that the use of 80 wt%Na₂CO₃/20 wt%MgO composite material successfully delayed the cooling time of the catalyst bed by 5–19 min in comparison to the case without a composite material. In addition, the Li₂CO₃/MgO and Na₂CO₃/MgO composite materials relatively revealed good performances: they prolonged the cooling time by over 10 min in the gas hourly space velocity (GHSV) range of 5000–12,500 h^?1. The application of the reactor tubes to solar tubular reformers is expected to realize stable operation of the solar reforming process under fluctuating insolation during cloud passage.     

Study of an adsorption refrigeration system powered by parabolic trough collector and coupled with a heat pipe

The aim of the current paper is to propose a study of a novel solar adsorptive cooling system, using activated carbon–ammonia pair, coupled with a parabolic trough collector (PTC) and a water-stainless steel heat pipe. A theoretical model, based on the thermodynamics of the adsorption process, heat and mass transfer within the porous medium and energy balance in the hybrid system components, is developed and a simulation code, written in FORTRAN, is carried out. This model, which has been validated by experimentation results, computes the temperature, pressure and adsorbed mass inside the adsorbent bed. The performance is assessed in terms of specific cooling power (SCP) and solar coefficient of performance (COPs). Furthermore, the effect of some important parameters on the system performance is discussed, and an optimization of these parameters is given.The simulation results have shown that there exists, for each aperture width value of the collector (W), an optimum external radius of adsorbent bed (R₂). Under the operating and design conditions of evaporation temperature T_ev = 0 °C, condensing temperature T_con = 28 °C, adsorption temperature T_ads = 24 °C, W = 0.70 m, R₂ = 0.145 m and reactor length of 0.5 m, an optimal corresponding COPs is found to be of the order of 0.18.     

Co-production of hydrogen,electricity and CO2 from coal with commercially ready technology. Part B: Economic analysis

This two-part paper investigates performances, costs and prospects of using commercially ready technology to convert coal to H₂ and electricity, with CO₂ capture and storage. Part A focuses on plant configuration, performance, and CO₂ emissions. Part B focuses on the cost of producing H₂ and electricity, with and without reduced CO₂ emissions. Our estimates show that the costs for $\sim$ 91% decarbonized energy (via quench gasification at 70 bar) are about $6.2¢/\mathrm{kWh}$ for electricity and about $ 1.0/kg (8.5  $/GJ, LHV) for hydrogen; these are, respectively, 35% and 19% higher than the corresponding energy costs with CO₂ venting. Referenced to these analogous CO₂ venting plants, the costs of CO₂ emissions avoided are $\sim$ 24 $/tonne for electricity and 11 $/tonne for H₂.