Is there an optimum level for renewable energy?

Because continued heavy use of fossil fuel will lead to both global climate change and resource depletion of easily accessible fuels, many researchers advocate a rapid transition to renewable energy (RE) sources. In this paper we examine whether RE can provide anywhere near the levels of primary energy forecast by various official organisations in a business-as-usual world. We find that the energy costs of energy will rise in a non-linear manner as total annual primary RE output increases. In addition, increasing levels of RE will lead to increasing levels of ecosystem maintenance energy costs per unit of primary energy output. The result is that there is an optimum level of primary energy output, in the sense that the sustainable level of energy available to the economy is maximised at that level. We further argue that this optimum occurs at levels well below the energy consumption forecasts for a few decades hence.     

中国中长期碳减排战略目标初探(Ⅴ)——非化石能源的需求与碳排放

随着技术的进步,非化石能源将成为重要的一次能源,其对化石能源的替代将对碳减排起到举足轻重的作用。核能属于不排碳的新能源,生物质能具有零排碳甚至负排碳的特点,其他可再生能源包括水能、风能和太阳能等均属清洁的超低排碳能源。先进的核能压水堆技术成熟,型式已发展到第三代,安全性不断提高。尽管发生了日本福岛核危机,但不应责怪核能本身,仍需要在高度重视安全的前提下大力发展核电。我国核电规模不宜过分压缩,2050年核电规模为300GW,发电量2100TW.h,占总用电量份额20%的目标比较恰当。用生物质秸秆制造燃料乙醇、用林业废弃物制造合成柴油和用废食用油脂制造生物柴油能显著降低运输业的排碳,7×108t干生物质可替代1×108t进口原油,在高油价时代其意义不言而喻。我国曾推广秸秆发电,但经济效益和社会效益有限,建议在生物质秸秆制造燃料乙醇达到产业化后不再扩大秸秆发电规模,以保证燃料乙醇产业的原料来源。水能、风能和太阳能发电量占总电量的比率是衡量绿色电力的重要指标,中国2050年这三种可再生能源发电量约3600TW.h,仅占总用电量的36%。非化石能源发电量占总发电量的份额是一项重要衡量指标,IEA在Blue Map等情景中将2050年该百分比定为61%～75%;中国工程院预测的我国2050年该百分比为70%。但如果2050年的核电装机容量为400GW时可提供总发电量的28%,加上水电、风电、太阳能三种可再生能源发电提供的36%,合计为64%,仍然存在6%的缺口;若核电装机容量只能达到300GW,缺口将达13%。如靠化石能源来弥补发电量的不足,将带来增加碳排放的后果。     

Solar energy integration into advanced building design for meeting energy demand and environment problem

The dangerous effects of burning fossil fuels on global warming, alternative energy sources will become indeed important in the future. Because of fossil fuels energy sources shall run out by the early 22nd century given the present rate of consumption. Atmospheric concentration of greenhouse gases is trapping heat radiated from the Earth's surface, which cause global warming and environmental problems such as greenhouses effect, stratospheric ozone depletion, acid precipitation, and flooding of coastal settlements. This implies that sooner or later humanity will rely heavily on renewable energy sources. Here we have introduced light energy at an idealized large‐scale application to produce solar energy, where exterior skin and roof of buildings shall be at least 25% blackbody‐assisted photovoltaic to capture solar energy during the whole year. Simply, it is a calculative reaction of solar irradiance on innovative building design to capture sunlight most efficiently that would be the cutting edge technology for the ultimate solution of global energy, environment, and climate crisis. Copyright © 2016 John Wiley & Sons, Ltd.     

Advanced electric power systems

An identification and preliminary evaluation was made of alternative advanced electric power systems which have been suggested for possible future use by the American electric utility industry. The motivation for interest in advanced power systems stems primarily from the rapidly rising costs of clean fossil fuels, especially conventional fuel oil, and uncertainties of fuel supply. Four basic energy sources have been identified for prospective future American utility applications; namely, coal, nuclear, solar and geothermal. Each source must generally be subjected to extensive preprocessing before thermal energy can be delivered in a form useful to an electric power conversion system. Numerous candidate advanced energy conversion systems can be matched to the various energy sources, including steam, open cycle gas turbines, combined cycles, closed cycle gas turbines, MHD, fuel cells, liquid metal topping, supercritical carbon dioxide topping and others. Each has advantages and disadvantages which can be ranked and weighted numerically, based on our present knowledge. A tentative selection of promising combinations of energy sources and conversion systems has been made to focus attention on those which satisfy the socio-political requirements and also offer potential profit opportunities for suppliers to the electric utility industry.     

Exergy analysis of renewable energy sources

Oil crises in the past years made more obvious the dependency of economies on fossil fuels. As a consequence, the need for new energy sources became more urgent. Renewable energy sources could provide a solution to the problem, as they are inexhaustible and have less adverse impacts on the environment than fossil fuels. Yet, renewable energy sources technology has not reached a high standard at which it can be considered competitive to fossil fuels. The present study deals with the exergy analysis of solar energy, wind power and geothermal energy. That is, the actual use of energy from the existing available energy is discussed. In addition, renewable energy sources are compared with the non-renewable energy sources on the basis of efficiency.     

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.     

Hydrogen systems for large-scale photovoltaic plants: Simulation with forecast and real production data

The unevenness of solar photovoltaic energy output poses a number of issues that reduce its capability to be considered a reliable substitute for fossil fuels. For instance, solar photovoltaic plants convert and inject energy in the grid during the daytime, but fail to do so during bad weather conditions or at night. Variable weather conditions also render a reliable energy injection planning impossible, causing the photovoltaic power plant output to be most often unpredictable. Furthermore, all the energy converted and immediately injected in the grid poses the risk of creating imbalances in the electric energy distribution lines. A nation-wide energy system characterized by a large penetration of photovoltaic and wind energy sources can therefore be extremely difficult to manage and cannot be considered dependable. The core issue is how to improve the reliability of electricity production from such renewable energy sources.     

Promotional policy and perspectives of usage renewable energy in Lithuania

The article outlines renewable energy (RE) sources according to the energy efficiency policy in Lithuania as well as practical experience of implementation of RE projects within the framework of the government policy to promote RES use due to the requirement of the European Union. The main goal of the country is to reduce the import of fossil fuel, to improve environment conditions and to reduce the climate change impact. Analysis of implemented RE projects and forecasts for the future projects are also presented. Most of the efforts in Lithuania were aimed at drafting the biomass (wood chips, wood waste, straw, biogas) and small hydro projects and their subsequent implementation. At present the total capacity of wood-chip-fuelled boilers reached above 251 MW. No serious obstacles can be seen for the extension of wood fuel use. At present, new demonstrational projects have been started covering geothermal energy, solar energy, biogas, biofuels for transport and other. In this time, the RE sources comprise 7.69% of national energy balance. Taking into account feasible resources of RE (it is more than 19.85 TWh/year) and the ongoing implementation of projects it is clear that the share of RE sources will constitute 12–13% of national energy balance in 2010 year. The main factor limiting further growth is high investment costs. The electricity production from local and RE sources in Lithuania is mainly based on hydro energy. At this time the wind energy is not used for this purpose. The electricity production from local and renewable energy sources is about 3.22% of the total consumption.     

Large-vscale hydrogen production and storage technologies: Current status and future directions

Over the past years, hydrogen has been identified as the most promising carrier of clean energy. In a world that aims to replace fossil fuels to mitigate greenhouse emissions and address other environmental concerns, hydrogen generation technologies have become a main player in the energy mix. Since hydrogen is the main working medium in fuel cells and hydrogen-based energy storage systems, integrating these systems with other renewable energy systems is becoming very feasible. For example, the coupling of wind or solar systems hydrogen fuel cells as secondary energy sources is proven to enhance grid stability and secure the reliable energy supply for all times. The current demand for clean energy is unprecedented, and it seems that hydrogen can meet such demand only when produced and stored in large quantities. This paper presents an overview of the main hydrogen production and storage technologies, along with their challenges. They are presented to help identify technologies that have sufficient potential for large-scale energy applications that rely on hydrogen. Producing hydrogen from water and fossil fuels and storing it in underground formations are the best large-scale production and storage technologies. However, the local conditions of a specific region play a key role in determining the most suited production and storage methods, and there might be a need to combine multiple strategies together to allow a significant large-scale production and storage of hydrogen.     

Nuclear power and oil imports : A look at the energy balance

There are still a great many arguments over the net energy benefits — if any — of replacing fossil fuel power stations with nuclear. Here the authors have used a simple model in an attempt to calculate the net energy costs of a number of possible strategies. They include that nuclear power can be effective in saving fossil fuels but that the saving is critically dependant both on the rate of reactor construction and the amount of energy needed to construct each reactor. For any given energy construction input there is an optimum rate of construction; programmes faster or slower than the optimum will result in less net energy output.     

What is the global potential for renewable energy?

World energy demand is projected to rise to 1000 EJ (EJ = 10¹⁸ J) or more by 2050 if economic growth continues its course of recent decades. Both reserve depletion and greenhouse gas emissions will necessitate a major shift from fossil fuels as the dominant energy source. Since nuclear power is now unlikely to increase its present modest share, renewable energy (RE) will have to provide for most energy in the future. This paper addresses the questions of what energy levels RE can eventually provide, and in what time frame. We find that when the energy costs of energy are considered, it is unlikely that RE can provide anywhere near a 1000 EJ by 2050. We further show that the overall technical potential for RE will fall if climate change continues. We conclude that the global shift to RE will have to be accompanied by large reductions in overall energy use for environmental sustainability.     

对我国风电发展战略的冷思考

我国风电发展迅速,计划2010年风电装机容量要达到3500×10^4kW,2020年达到1.5×10^8kW。据IEA预测,2030年世界能源供应仍以化石能源为主,其比重由2006年的80.8%下降到80.4%;2030年世界发电能源结构也以化石能源发电为主,其比重由2006年的74%下降到73%。中国到21世纪中叶传统化石能源仍将居绝对优势地位。因此在可再生能源和新能源的开发过程中,不要急于求成,片面追求能源和电源结构优化不可取。我国未来要依靠核电和新能源发电,但需要通过对其技术经济的进一步研究,才能确定主要靠核电还是风电、太阳能发电或生物质能发电。目前我国风电发展的主要问题是对风电的技术要求起点低,技术路线不对,从国外引进了落后的风电技术。为了我国风电的健康发展,必须加快风电合理利用的研究,包括风电储能和风电直接利用的研究。     

Solar thermal hybrids for combustion power plant: A growing opportunity

The development of technologies to hybridise concentrating solar thermal energy (CST) and combustion technologies, is driven by the potential to provide both cost-effective CO₂ mitigation and firm supply. Hybridisation, which involves combining the two energy sources within a single plant, offers these benefits over the stand-alone counterparts through the use of shared infrastructure and increased efficiency. In the near-term, hybrids between solar and fossil fuelled systems without carbon capture offer potential to lower the use of fossil fuels, while in the longer term they offer potential for low-cost carbon-neutral or carbon-negative energy. The integration of CST into CO₂ capture technologies such as oxy-fuel combustion and chemical looping combustion is potentially attractive because the same components can be used for both CO₂ capture and the storage of solar energy, to reduce total infrastructure and cost. The use of these hybrids with biomass and/or renewable fuels, offers the additional potential for carbon-negative energy with relatively low cost. In addition to reviewing these technologies, we propose a methodology for classifying solar-combustion hybrid technologies and assess the progress and challenges of each. Particular attention is paid to “direct hybrids”, which harness the two energy sources in a common solar receiver or reactor to reduce total infrastructure and losses.     

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 transition to renewables: Can PV provide an answer to the peak oil and climate change challenges?

This paper explores energy and physical resource limitations to transitioning from fossil fuels to the large-scale generation of electricity with photovoltaic arrays. The model finds that business as usual models, which involve growth rates in world electricity demand of between 2% and 3.2% p.a., exhibit severe material difficulties before the end of this century. If the growth rate is lowered to 1% p.a., then it may be possible to reach the year 2100 before such difficulties, but it is likely that material constraints will occur early the next century. Steady state scenarios show that silicon based photovoltaic panels could, however, displace fossil fuels before the middle of the century, providing around the same order of magnitude as present (2010) world electricity demand. Scenarios also show that outcomes will be highly dependent upon the rate of improvement of photovoltaic technologies. The analysis does not contend that silicon PV technology is the only technology that will or can be adopted, but as the embodied energy content per kWh generated of this technology is similar to other renewable technologies, such as other solar technologies and wind, it can provide a baseline for examining a transition to a mixture of renewable energy sources.     

Potential of renewable hydrogen production for energy supply in Hong Kong

Hong Kong is highly vulnerable to energy and economic security due to the heavy dependence on imported fossil fuels. The combustion of fossil fuels also causes serious environmental pollution. Therefore, it is important to explore the opportunities for clean renewable energy for long-term energy supply. Hong Kong has the potential to develop clean renewable hydrogen energy to improve the environmental performance. This paper reviews the recent development of hydrogen production technologies, followed by an overview of the renewable energy sources and a discussion about potential applications for renewable hydrogen production in Hong Kong. The results show that although renewable energy resources cannot entirely satisfy the energy demand in Hong Kong, solar energy, wind power, and biomass are available renewable sources for significant hydrogen production. A system consisting of wind turbines and photovoltaic (PV) panels coupled with electrolyzers is a promising design to produce hydrogen. Biomass, especially organic waste, offers an economical, environmental-friendly way for renewable hydrogen production. The achievable hydrogen energy output would be as much as 40% of the total energy consumption in transportation.     

Impact of hydrogen on the environment

The present work considers the impact of hydrogen fuel on the environment within the cycles of its generation and combustion. Hydrogen has been portrayed by the media as a fuel that is environmentally clean because its combustion results in the formation of harmless water. However, hydrogen first must be generated. The effect of hydrogen generation on the environment depends on the production process and the related by-products. Hydrogen available on the market at present is mainly generated by using steam reforming of natural gas, which is a fossil fuel. Its by-product is CO₂, which is a greenhouse gas and its emission results in global warming and climate change. Therefore, hydrogen generated from fossil fuels is contributing to global warming to the similar extent as direct combustion of the fossil fuels. On the other hand hydrogen obtained from renewable energy, such solar energy, is environmentally clean during the cycles of its generation and combustion. Consequently, the introduction of hydrogen economy must be accompanied by the development of hydrogen that is environmentally friendly. The present work considers several aspects related to the generation and utilisation of hydrogen obtained by steam reforming and solar energy conversion (solar-hydrogen).     

Steps toward the hydrogen economy

The hydrogen economy is defined as the industrial system in which one of the universal energy carriers is hydrogen (the other is electricity) and hydrogen is oxidized to water that may be reused by applying an external energy source for dissociation of water into its component elements hydrogen and oxygen. There are three different primary energy-supply system classes which may be used to implement the hydrogen economy, namely, fossil fuels (coal, petroleum, natural gas, and as yet largely unused supplies such as shale oil, oil from tar sands, natural gas from geo-pressured locations, etc.), nuclear reactors including fission reactors and breeders or fusion nuclear reactors over the very long term, and renewable energy sources (including hydroelectric power systems, wind-energy systems, ocean thermal energy conversion systems, geothermal resources, and a host of direct solar energy-conversion systems including biomass production, photovoltaic energy conversion, solar thermal systems, etc.). Examination of present costs of hydrogen production by any of these means shows that the hydrogen economy favored by people searching for a non-polluting gaseous or liquid energy carrier will not be developed without new discoveries or innovations. Hydrogen may become an important market entry in a world with most of the electricity generated in nuclear fission or breeder reactors when high-temperature waste heat is used to dissociate water in chemical cycles or new inventions and innovations lead to low-cost hydrogen production by applying as yet uneconomical renewable solar techniques that are suitable for large-scale production such as direct water photolysis with suitably tailored band gaps on semiconductors or low-cost electricity supplies generated on ocean-based platforms using temperature differences in the tropical seas.     

Multi-objective optimal design of hybrid renewable energy systems using PSO-simulation based approach

Recently, the increasing energy demand has caused dramatic consumption of fossil fuels and unavoidable raising energy prices. Moreover, environmental effect of fossil fuel led to the need of using renewable energy (RE) to meet the rising energy demand. Unpredictability and the high cost of the renewable energy technologies are the main challenges of renewable energy usage. In this context, the integration of renewable energy sources to meet the energy demand of a given area is a promising scenario to overcome the RE challenges. In this study, a novel approach is proposed for optimal design of hybrid renewable energy systems (HRES) including various generators and storage devices. The ε-constraint method has been applied to minimize simultaneously the total cost of the system, unmet load, and fuel emission. A particle swarm optimization (PSO)-simulation based approach has been used to tackle the multi-objective optimization problem. The proposed approach has been tested on a case study of an HRES system that includes wind turbine, photovoltaic (PV) panels, diesel generator, batteries, fuel cell (FC), electrolyzer and hydrogen tank. Finally, a sensitivity analysis study is performed to study the sensibility of different parameters to the developed model.