Integrated fossil fuel and solar thermal systems for hydrogen production and CO2 mitigation

In most current fossil-based hydrogen production methods, the thermal energy required by the endothermic processes of hydrogen production cycles is supplied by the combustion of a portion of the same fossil fuel feedstock. This increases the fossil fuel consumption and greenhouse gas emissions. This paper analyzes the thermodynamics of several typical fossil fuel-based hydrogen production methods such as steam methane reforming, coal gasification, methane dissociation, and off-gas reforming, to quantify the potential savings of fossil fuels and CO₂ emissions associated with the thermal energy requirement. Then matching the heat quality and quantity by solar thermal energy for different processes is examined. It is concluded that steam generation and superheating by solar energy for the supply of gaseous reactants to the hydrogen production cycles is particularly attractive due to the engineering maturity and simplicity. It is also concluded that steam-methane reforming may have fewer engineering challenges because of its single-phase reaction, if the endothermic reaction enthalpy of syngas production step (CO and H₂) of coal gasification and steam methane reforming is provided by solar thermal energy. Various solar thermal energy based reactors are discussed for different types of production cycles as well.     

Development and environmental impact of hydrogen supply chain in Japan: Assessment by the CGE-LCA method in Japan with a discussion of the importance of biohydrogen

Hydrogen is an attractive and clean source of energy with a high energy content and environmentally friendly production using green power. Hydrogen is therefore considered to be one of the future alternatives to fossil fuels that can limit the damage done by climate change. A dynamic GTAP model with LCA method is utilized herein in this investigation to forecast the development of the hydrogen supply chain and CO₂ emissions in Japan. The supply chain incorporates six hydrogen-related industries – biohydrogen, steam reforming, electrolysis, hydrogen fuel cell vehicles (HFCV), hydrogen fuel cells (HFC), and hydrogen fueling stations.     

Transition to hydrogen economy in the United States: A 2006 status report

Energy crises in the latter part of the 20th century, as well as the current increase in the cost of oil, emphasize the need for alternate sources of energy in the United States. Concerns about climate change dictate that the source be clean and not contribute to global warming. Hydrogen has been identified as such a source for many years and the transition to a hydrogen economy was predicted to occur from the mid-1970s to 2000. This paper reports on the status of this transition in the year 2006. Instead of being a clean source of energy, most of the hydrogen produced in the US results from steam reforming of fossil fuels, releasing _CO2${\mathrm{CO}}_2$ and other pollutants to the atmosphere. Nuclear process heat is ideally suited for the production of hydrogen, either using electricity for electrolysis of water, or heat for thermochemical hydrogen production or reforming of fossil fuels. However, no new nuclear plants have been ordered or built in the United States since 1979, and it may be many years before high-temperature nuclear reactors are available for production of hydrogen. Considerable research and development efforts are focused on commercializing hydrogen-powered vehicles to lessen the dependence of the transportation sector on imported oil. However, the use of hydrogen fuel cell vehicles (FCV) in 2006 is two orders-of-magnitude less than what has been predicted. Although it makes little sense environmentally or economically, hydrogen is also used as fuel in internal combustion engines. Development of hydrogen economy will require a strong intervention by external forces.     

Potential importance of hydrogen as a future solution to environmental and transportation problems

Air pollution is a serious public health problem throughout the world, especially in industrialized and developing countries. In industrialized and developing countries, motor vehicle emissions are major contributors to urban air quality. Hydrogen is one of the clean fuel options for reducing motor vehicle emissions. Hydrogen is not an energy source. It is not a primary energy existing freely in nature. Hydrogen is a secondary form of energy that has to be manufactured like electricity. It is an energy carrier. Hydrogen has a strategic importance in the pursuit of a low-emission, environment-benign, cleaner and more sustainable energy system. Combustion product of hydrogen is clean, which consists of water and a little amount of nitrogen oxides. Hydrogen has very special properties as a transportation fuel, including a rapid burning speed, a high effective octane number, and no toxicity or ozone-forming potential. It has much wider limits of flammability in air than methane and gasoline. Hydrogen has become the dominant transport fuel, and is produced centrally from a mixture of clean coal and fossil fuels (with C-sequestration), nuclear power, and large-scale renewables. Large-scale hydrogen production is probable on the longer time scale. In the current and medium term the production options for hydrogen are first based on distributed hydrogen production from electrolysis of water and reforming of natural gas and coal. Each of centralized hydrogen production methods scenarios could produce 40 million tons per year of hydrogen. Hydrogen production using steam reforming of methane is the most economical method among the current commercial processes. In this method, natural gas feedstock costs generally contribute approximately 52–68% to the final hydrogen price for larger plants, and 40% for smaller plants, with remaining expenses composed of capital charges. The hydrogen production cost from natural gas via steam reforming of methane varies from about 1.25 US$/kg for large systems to about 3.50 US$/kg for small systems with a natural gas price of 6 US$/GJ. Hydrogen is cheap by using solar energy or by water electrolysis where electricity is cheap, etc.     

Political,economic and environmental impacts of biomass-based hydrogen

The purpose of this study is to assess the political, economic and environmental impacts of producing hydrogen from biomass. Hydrogen is a promising renewable fuel for transportation and domestic applications. Hydrogen is a secondary form of energy that has to be manufactured like electricity. The promise of hydrogen as an energy carrier that can provide pollution-free, carbon-free power and fuels for buildings, industry, and transport makes it a potentially critical player in our energy future. Currently, most hydrogen is derived from non-renewable resources by steam reforming in which fossil fuels, primarily natural gas, but could in principle be generated from renewable resources such as biomass by gasification. Hydrogen production from fossil fuels is not renewable and produces at least the same amount of CO₂ as the direct combustion of the fossil fuel. The production of hydrogen from biomass has several advantages compared to that of fossil fuels. The major problem in utilization of hydrogen gas as a fuel is its unavailability in nature and the need for inexpensive production methods. Hydrogen production using steam reforming methane is the most economical method among the current commercial processes. These processes use non-renewable energy sources to produce hydrogen and are not sustainable. It is believed that in the future biomass can become an important sustainable source of hydrogen. Several studies have shown that the cost of producing hydrogen from biomass is strongly dependent on the cost of the feedstock. Biomass, in particular, could be a low-cost option for some countries. Therefore, a cost-effective energy-production process could be achieved in which agricultural wastes and various other biomasses are recycled to produce hydrogen economically. Policy interest in moving towards a hydrogen-based economy is rising, largely because converting hydrogen into useable energy can be more efficient than fossil fuels and has the virtue of only producing water as the by-product of the process. Achieving large-scale changes to develop a sustained hydrogen economy requires a large amount of planning and cooperation at national and international alike levels.     

A review on biomass based hydrogen production technologies

Hydrogen is a renewable energy carrier that is one of the most competent fuel options for the future. The majority of hydrogen is currently produced from fossil fuels and their derivatives. These technologies have a negative impact on the environment. Furthermore, these resources are rapidly diminishing. Recent research has focused on environmentally friendly and pollution-free alternatives to fossil fuels. The advancement of bio-hydrogen technology as a development of new sustainable and environmentally friendly energy technologies was examined in this paper. Key chemical derivatives of biomass such as alcohols, glycerol, methane-based reforming for hydrogen generation was briefly addressed. Biological techniques for producing hydrogen are an appealing and viable alternative. For bio-hydrogen production, these key biological processes, including fermentative, enzymatic, and biocatalyst, were also explored. This paper also looks at current developments in the generation of hydrogen from biomass. Pretreatment, reactor configuration, and elements of genetic engineering were also briefly covered. Bio-H₂ production has two major challenges: a poor yield of hydrogen and a high manufacturing cost. The cost, benefits, and drawbacks of different hydrogen generation techniques were depicted. Finally, this article discussed the promise of biohydrogen as a clean alternative, as well as the areas in which additional study is needed to make the hydrogen economy a reality.     

Solar decomposition of fossil fuels as an option for sustainability

This paper presents an overview on solar-thermal decomposition of fossil fuels as a viable option for transition path from today's permanent dependency on fossil fuels to tomorrow's solar fuels via solar thermochemical technology. The paper focuses on the thermochemical hydrogen generation technologies from concentrated solar energy and gives an assessment of the recent advancements in the hydrogen producing solar reactors. The advantages and obstacles of hydrogen generation via solar cracking and solar reforming are presented along with some discussions on the feasibility of industrial scaling of these technologies. Solar cracking and solar reforming processes are discussed as promising hybrid solar/fossil technologies to take considerable share during transition from fossil fuel dependency to clean energy based sustainability.     

Generation of green hydrogen using self-sustained regenerative fuel cells: Opportunities and challenges

The ever-increasing energy demand, depleting fossil fuel reserves, and rising temperatures due to greenhouse gas emissions have necessitated the transition towards the generation of green and clean energy through renewable energy sources. Solar energy is one such renewable energy source that has received significant attention owing to its abundance and inexhaustibility. However, solar energy alone cannot replace fossil fuels in the energy portfolio. There exists a need to develop another clean energy source that can potentially act as an alternative to conventional fuels. Hydrogen proves to be an ideal candidate in this domain and can be sustainably generated by water electrolysis by powering the electrolyzer using solar energy. The hydrogen thus synthesized has net zero carbon emissions and is a suitable asset for decarbonizing the environment. This review encompasses the generation of hydrogen using PV-Electrolyzer systems and addresses the challenges associated with the same. Overcoming these drawbacks can ensure a strong position for hydrogen as an alternative fuel in the energy infrastructure. By employing electrolyzers that are fueled by renewable energy and then using that hydrogen to feed a fuel cell, this study aims to clarify the potential and constraints of producing green hydrogen. Since this area of research has not yet been fully investigated, a review article that enables and encourages academics to develop original solutions is urgently needed.     

Well-to-Wheels analysis of hydrogen production from bio-oil reforming for use in internal combustion engines

The environmental profile of hydrogen depends greatly on the nature of the feedstock and the production process. In this Well-to-Wheels (WTW) study, the environmental impacts of hydrogen production from lignocellulosic biomass via pyrolysis and subsequent steam reforming of bio-oil were evaluated and compared to the conventional production of hydrogen from natural gas steam reforming. Hydrogen was assumed to be used as transportation fuel in an internal combustion engine vehicle. Two scenarios for the provision of lignocellulosic biomass were considered: wood waste and dedicated willow cultivation. The WTW analysis showed that the production of bio-hydrogen consumes less fossil energy in the total lifecycle, mainly due to the renewable nature of the fuel that results in zero energy consumption in the combustion step. The total (fossil and renewable) energy demand is however higher compared to fossil hydrogen, due to the higher process energy demands and methanol used to stabilize bio-oil. Improvements could occur if these are sourced from renewable energy sources. The overall benefit of using a CO₂ neutral renewable feedstock for the production of hydrogen is unquestionable. In terms of global warming, production of hydrogen from biomass through pyrolysis and reforming results in major GHG emissions, ranging from 40% to 50%, depending on the biomass source. The use of cultivated biomass aggravates the GHG emissions balance, mainly due to the N₂O emissions at the cultivation step.     

Green hydrogen as feedstock: Financial analysis of a photovoltaic-powered electrolysis plant

The weather-dependent electricity generation from Renewable Energy Sources (RES), such as solar and wind power, entails that systems for energy storage are becoming progressively more important. Among the different solutions that are being explored, hydrogen is currently considered as a key technology allowing future long-term and large-scale storage of renewable power.Today, hydrogen is mainly produced from fossil fuels, and steam methane reforming (SMR) is the most common route for producing it from natural gas. None of the conventional methods used is GHG-free. The Power-to-Gas concept, based on water electrolysis using electricity coming from renewable sources is the most environmentally clean approach. Given its multiple uses, hydrogen is sold both as a fuel, which can produce electricity through fuel cells, and as a feedstock in several industrial processes. Just the feedstock could be, in the short term, the main market of RES-based hydrogen.In this paper, we present the results obtained from a techno-economic-financial evaluation of a system to produce green hydrogen to be sold as a feedstock for industries and research centres. A system which includes a 200 kW photovoltaic plant and a 180 kW electrolyser, to be located in Messina (Italy), is proposed as a case study. According to the analyses carried out, and taking into account the current development of technologies, it has been found that investment to realise a small-scale PV-based hydrogen production plant can be remunerative.     

Emerging technologies by hydrogen: A review

The majority of energy being used is obtained from fossil fuels, which are not renewable resources and require a longer time to recharge or return to its original capacity. Energy from fossil fuels is cheaper but it faces some challenges compared to renewable energy resources. Thus, one of the most potential candidates to fulfil the energy requirements are renewable resources and the most environmentally friendly fuel is Hydrogen. Hydrogen is a clean and efficient energy carrier and a hydrogen-based economy is now widely regarded as a potential solution for the future of energy security and sustainability. Hydrogen energy became the most significant energy as the current demand gradually starts to increase. It is an important key solution to tackle the global temperature rise. The key important factor of hydrogen production is the hydrogen economy. Hydrogen production technologies are commercially available, while some of these technologies are still under development. Therefore, the global interest in minimising the effects of greenhouse gases as well as other pollutant gases also increases. In order to investigate hydrogen implementation as a fuel or energy carrier, easily obtained broad-spectrum knowledge on a variety of processes is involved as well as their advantages, disadvantages, and potential adjustments in making a process that is fit for future development. Aside from directly using the hydrogen produced from these processes in fuel cells, streams rich with hydrogen can also be utilised in producing ethanol, methanol, gasoline as well as various chemicals of high value. This paper provided a brief summary on the current and developing technologies of hydrogen that are noteworthy.     

Development of Oshawa hydrogen hub in Canada: A case study

Consumption of fossil fuels, which makes an immense contribution to greenhouse gas emissions, must be reduced. Hydrogen emerges as a unique solution to serve as fuel, energy carrier and feedstock because it is a clean, abundant, environmentally friendly and energy intensive gas. This study aims to investigate the development of a potential hydrogen hub located in Oshawa, Canada, which is aimed to provide a hydrogen infrastructure for future hydrogen economy. Numerous life cycle assessment and cost assessment studies are conducted to investigate what benefits such a hydrogen will bring to the city. The results show that fuel cell electric buses emit 89% fewer pollutants. Also, 60% of overall CO₂ reduction is possible with a gradual transition to fuel cell technology within 20 years. However, in order for hydrogen infrastructure and costs to compete with fossil fuels, high-scale projects need to be developed with governmental incentives.     

Public perception related to a hydrogen hybrid internal combustion engine transit bus demonstration and hydrogen fuel

Hydrogen has been widely considered as a potentially viable alternative to fossil fuels for use in transportation. In addition to price competitiveness with fossil fuels, a key to its adoption will be public perceptions of hydrogen technologies and hydrogen fuel. This paper examines public perceptions of riders of a hydrogen hybrid internal combustion engine bus and hydrogen as a fuel source.     

Eco-efficiency of H2 and fuel cell buses

Urban governments are continually striving to improve air quality by making public transportation more environmentally friendly. H₂ fuel cell buses (FCBs) offer one of the best ways to reduce air pollution. FCB has high energy efficiency and lower air pollutant emissions than conventional buses (e.g. diesel bus/Compressed natural gas bus, CNGB), and H₂ is an attractive alternative energy source in the face of depleting fossil fuels and global warming. H₂ can be produced via fossil fuels and renewable sources and then stored and distributed in a variety of different ways. While many contend that H₂ and FCB are not yet commercially viable, H₂ technology has developed a great deal over recent years. This fact alone demands that governments as well as for-profit businesses take a discerning look at what H₂ and FCB have to offer in terms of both environmental and economic opportunities.In this study, environmental and economic aspects of hydrogen pathways are analyzed according to plausible production methods and capacity, and distribution options in Korea using life cycle assessment (LCA) and life cycle costing (LCC) methods. This study considers the following means of hydrogen production: naphtha steam reforming (Naphtha SR), natural gas steam reforming (NG SR), and water electrolysis (WE). Additionally, conventional fuels (Diesel and CNG) are also included as target fuel pathways in order to identify which hydrogen pathway in particular has the greatest environmental advantage over conventional fuels. This study aimed to identify whether H₂ and FCB can compete with conventional fuels used in buses in terms of the eco-efficiency method, which focuses on economic feasibility and environmental improvement.The conclusion of this study is that H₂ pathways, especially, Naphtha SR [C] and NG SR [S], are more competitive than conventional fuels from an eco-efficiency perspective. As a result, switching from conventional transportation fuel to these suggested H₂ pathways is expected to offer an economically and environmentally more eco-efficient means of transportation. Henceforth, drawing upon evidence within this report, decision-makers would be wise to invest in more cost-effective and environment-friendly fuels by constructing an optimal H₂ infrastructure.     

Carbon black and hydrogen production process analysis

The adoption of new environmentally responsible technologies, as well as, energy efficiency improvements in equipment and processes help to reduce CO2 rate emission into the atmosphere, contributing in delaying the consequences of intensive use of fossil fuels. For more effective actions, it is necessary to make the transition from the fossil-based to the renewable source economy. In this context, hydrogen fuel has a special role as clean vector of energy. Hydrogen has the potential to be decisive in mitigating greenhouse gas emissions, but fossil fuels high profitability due to global energy dependency actually drives the global economy.While renewable energy sources are not worldwide fully established, new technologies should be developed and used for the recovery of energetic streams nowadays wasted, to decarbonize hydrocarbons and to improve systems efficiency creating a path that can help nations and industries in the needed energy economy transition. Hydrogen gas can be generated by various methods from different sources such as coal and water. Currently, almost all of the hydrogen production is for industrial purpose and comes from the Steam Reforming, while the use of hydrogen in fuel cells is only incipient.The article analysis the plasma pyrolysis of hydrocarbons as a decarbonization option to contribute as a step towards hydrogen economy. It presents the Carbon Black and Hydrogen Process (CB&H Process) as an alternative option for hydrogen generation at large scale facility, suitable for supplying large amounts of high-purity carbon in elemental form. CB&H Process refers to a plant with hydrogen thermal plasma reactor able to decompose Hydrocarbons (HC's) into Hydrogen (H2) and Carbon Black (CB), a cleaner technology than its competing processes, capable of generating two products with high added value. Considering the Brazilian context in which more than 80% of the generated electricity comes from renewable sources, the use of electricity as one of the inputs in the process does not compromise the objective of reducing greenhouse gas emissions. It is important to consider that the use of renewable energy to produce two products derived from fossil fuels in a clean way represents integration of technologies into a more efficient system and an arrangement that contributes to the transition from fossil fuels to renewables.The economic viability of the CB&H process as a hydrogen generation unit (centralized) for refining applications also depends on the cost of hydrogen production by competing processes. Steam Methane Reforming (SMR) is a widespread method that produces twice the amount of hydrogen generated by natural gas plasma pyrolysis, but it emits CO2 gas and consumes water, while CB&H process produces solid carbon. For this reason, the paper seeks the carbon production cost by plasma pyrolysis as a breakeven point for large-scale hydrogen generation without water consumption and carbon dioxide emissions.     

Investigation of biomass gasification hydrogen and electricity co-production with carbon dioxide capture and storage

Biomass is one of the renewable energy resources which can be used instead of fossil fuels to diminish environment pollution and emission of greenhouse gases. Hydrogen as a biomass is considered as an alternative fuel which can be derived from a variety of domestically available primary sources. In this paper, a hydrogen and electricity co-generation plant with rice husk is proposed. Rice husk with water vapor and oxygen produces syngas in gasifier. In this design, electricity is generated by using two Rankine cycles. The Results show that the net electric efficiency and hydrogen production efficiency are 1.5% and 40.0%, respectively. Hydrogen production is 1.316 kg/s in case which carbon dioxide is gathered and stored. The electricity generation is 5.923 MW_e. The main propose of implementing Rankine cycle is to eliminate hydrogen combustion for generating electricity and to reduce NO_x production. Furthermore, three kinds of membranes are studied in this paper.     

Simulating the steam reforming of sunflower meal in Aspen Plus

Hydrogen is a clean energy carrier that has the potential to mitigate the environmentally hazardous effects of fossil fuels. Hydrogen is mainly produced through the steam reforming of natural gas however it is also possible to produce hydrogen through the thermochemical conversion of various biomasses. In this study, three Aspen plus simulation models were developed to obtain hydrogen-rich gas products from biomass through catalytic steam reforming. The results obtained in this modeling study were compared to the experimental data obtained by the steam reforming of the sunflower meal, which is a waste product of the seed oil industry. Out of all three models, model II, in which all of the reactions are assumed to occur simultaneously and all species except for biomass are assumed to undergo combustion reactions, was the most successful one at predicting close results (93% similar) to experimental findings. Using this model, the effect of water:biomass feed ratio on the product yields was tested and the highest possible H₂ yield (44.9 mol H₂/kg sunflower meal) was achieved with a 15:1 water:biomass feed ratio at the constant temperature of 800 °C and atmospheric pressure.     

Hydrogen from solar energy,a clean energy carrier from a sustainable source of energy

Solar energy is going to play a crucial role in the future energy scenario of the world that conducts interests to solar-to-hydrogen as a means of achieving a clean energy carrier. Hydrogen is a sustainable energy carrier, capable of substituting fossil fuels and decreasing carbon dioxide (CO₂) emission to save the world from global warming. Hydrogen production from ubiquitous sustainable solar energy and an abundantly available water is an environmentally friendly solution for globally increasing energy demands and ensures long-term energy security. Among various solar hydrogen production routes, this study concentrates on solar thermolysis, solar thermal hydrogen via electrolysis, thermochemical water splitting, fossil fuels decarbonization, and photovoltaic-based hydrogen production with special focus on the concentrated photovoltaic (CPV) system. Energy management and thermodynamic analysis of CPV-based hydrogen production as the near-term sustainable option are developed. The capability of three electrolysis systems including alkaline water electrolysis (AWE), polymer electrolyte membrane electrolysis, and solid oxide electrolysis for coupling to solar systems for H₂ production is discussed. Since the cost of solar hydrogen has a very large range because of the various employed technologies, the challenges, pros and cons of the different methods, and the commercialization processes are also noticed. Among three electrolysis technologies considered for postulated solar hydrogen economy, AWE is found the most mature to integrate with the CPV system. Although substantial progresses have been made in solar hydrogen production technologies, the review indicates that these systems require further maturation to emulate the produced grid-based hydrogen.     

The potential role of hydrogen as a sustainable transportation fuel to combat global warming

Hydrogen is recognized as a key source of the sustainable energy solutions. The transportation sector is known as one of the largest fuel consumers of the global energy market. Hydrogen can become a promising fuel for sustainable transportation by providing clean, reliable, safe, convenient, customer friendly, and affordable energy. In this study, the possibility of hydrogen as the major fuel for transportation systems is investigated comprehensively based on the recent data published in the literature. Due to its several characteristic advantages, such as energy density, abundance, ease of transportation, a wide variety of production methods from clean and renewable fuels with zero or minimal emissions; hydrogen appears to be a great chemical fuel which can potentially replace fossil fuel use in internal combustion engines. In order to take advantage of hydrogen as an internal combustion engine fuel, existing engines should be redesigned to avoid abnormal combustion. Hydrogen use in internal combustion engines could enhance system efficiencies, offer higher power outputs per vehicle, and emit lower amounts of greenhouse gases. Even though hydrogen-powered fuel cells have lower emissions than internal combustion engines, they require additional space and weight and they are generally more expensive. Therefore, the scope of this study is hydrogen-fueled internal combustion engines. It is also highlighted that in order to become a truly sustainable and clean fuel, hydrogen should be produced from renewable energy and material resources with zero or minimal emissions at high efficiencies. In addition, in this study, conventional, hybrid, electric, biofuel, fuel cell, and hydrogen fueled ICE vehicles are comparatively assessed based on their CO₂ and SO₂ emissions, social cost of carbon, energy and exergy efficiencies, fuel consumption, fuel price, and driving range. The results show that when all of these criteria are taken into account, fuel cell vehicles have the highest average performance ranking (4.97/10), followed by hydrogen fueled ICEs (4.81/10) and biofuel vehicles (4.71/10). On the other hand, conventional vehicles have the lowest average performance ranking (1.21/10), followed by electric vehicles (4.24/10) and hybrid vehicles (4.53/10).     

Screening of water-splitting thermochemical cycles potentially attractive for hydrogen production by concentrated solar energy

Hydrogen, a promising and clean energy carrier, could potentially replace the use of fossil fuels in the transportation sector. Currently, no environmentally attractive, large-scale, low-cost and high-efficiency hydrogen production process is available for commercialization. Solar-driven water-splitting thermochemical cycles may constitute one of the ultimate options for CO₂-free production of hydrogen. The method is environmentally friendly since it uses only water and solar energy. First, the potentially attractive thermochemical cycles must be identified based on a set of criteria. To reach this goal, a database that contains 280 referenced cycles was established. Then, the selection and evaluation of the promising cycles was performed in the temperature range of 900–2000 °C, suitable to the use of concentrated solar energy. About 30 cycles selected for further investigations are presented in this paper. The principles and basis for a thermodynamic evaluation of the cycles are also given.