Showing papers on "Thermal energy published in 2015"

Heating and cooling energy trends and drivers in buildings

Abstract: The purpose of this paper is to provide a source of information on thermal energy use in buildings, its drivers, and their past, present and future trends on a global and regional basis. Energy use in buildings forms a large part of global and regional energy demand. The importance of heating and cooling in total building energy use is very diverse with this share varying between 18% and 73%. Biomass is still far the dominant fuel when a global picture is considered; the role of electricity is substantially growing, and the direct use of coal is disappearing from this sector, largely replaced by electricity and natural gas in the most developed regions. This paper identifies the different drivers of heating and cooling energy demand, and decomposes this energy demand into key drivers based on a Kaya identity approach: number of households, persons per household, floor space per capita and specific energy consumption for residential heating and cooling; and GDP, floor space per GDP, and specific energy consumption for commercial buildings. This paper also reviews the trends in the development of these drivers for the present, future – and for which data were available, for the past – in 11 world regions as well as globally. Results show that in a business-as-usual scenario, total residential heating and cooling energy use is expected to more or less stagnate, or slightly decrease, in the developed parts of the world. In contrast, commercial heating and cooling energy use will grow in each world region. Finally, the results show that per capita total final residential building energy use has been stagnating in the vast majority of world regions for the past three decades, despite the very significant increases in energy service levels in each of these regions.

A review of promising candidate reactions for chemical heat storage

Abstract: Thermal energy storage is a necessary technology for the application of renewable energy and low-grade thermal energy. Chemical heat storage has been proved to be a feasible and promising method to store thermal energy. As compared to other thermal energy storage methods, chemical heat storage exhibits high energy storage density as well as feasibility for long-duration energy storage. In this paper, the basic principle of the chemical heat storage is firstly elaborated. Then the selection criteria of the chemical reaction are given. The aim of this review is to provide an insight into the promising candidate reactions for chemical heat storage application. The associated reversible chemical reactions available for thermal energy storage systems are summarized. Ongoing research and development studies illustrate that chemical heat storage is a very favorable option for the different application when diverse promising candidate reactions are selected. As working temperature is one of the key parameters for thermal energy storage systems, emphasis is given to the judgment of application temperature range for chemical heat storage. The determination of applicative temperature range of reversible chemical reactions is discussed. Besides, the challenge and prospect of the chemical heat storage technology are analyzed in the paper.

Review of the application of phase change material for heating and domestic hot water systems

Abstract: Heating and domestic hot water (DHW) systems account for 75% of energy consumption in residential, commercial, and industrial sectors. Furthermore, thermal energy storage strongly reduces energy consumption. Storage devices of thermal energy from phase change material (PCM) are essential in solar thermal and waste heat energy technologies that match energy supply to demand and enhance their thermal performance. The storage of PCM thermal energy is more beneficial than sensible energy storage because of its high density of storage energy per unit volume/mass. This review presents previous works on thermal energy storage as applied to DHW and heating systems. PCM has been used in different parts of heating networks and DHW systems, including solar collectors, storage tanks, packed beds, and duct networks. Researchers have also investigated the application of PCM in heating and DHW systems to reduce greenhouse gas emission and electrical power consumption. Hence, PCM thermal energy storage is expected to lower cost and the volumes of heating and DHW systems.

Arbitraging Intraday Wholesale Energy Market Prices With Aggregations of Thermostatic Loads

Abstract: We investigate the potential for aggregations of residential thermostatically controlled loads (TCLs), such as air conditioners, to arbitrage intraday wholesale electricity market prices via non-disruptive load control. We present two arbitrage approaches: 1) a benchmark that gives us an optimal policy but requires local computation or real-time communication and 2) an alternative based on a thermal energy storage model, which relies on less computation/communication infrastructure, but is suboptimal. We find that the alternative approach achieves around 60%–80% of the optimal wholesale energy cost savings. We use this approach to compute practical upper bounds for savings via arbitrage with air conditioners in California's intraday energy market. We investigate six sites over four years and find that the savings range from $2–$37 per TCL per year, and depend upon outdoor temperature statistics and price volatility.

Catalog of CHP Technologies

Abstract: This report is a survey of technologies related to combined heat and power (CHP) which is an efficient and clean approach to generating electric power and useful thermal energy from a single fuel source.

A thermodynamic review of solar air heaters

Abstract: Solar air heaters (SAHs) form the foremost component of solar energy utilization system. These air heaters absorb the irradiance and convert it into thermal energy at the absorbing surface and then transfer this energy to a fluid flowing through the collector. SAHs are inexpensive and most used collection devices because of their inherent simplicity. SAHs are found in several solar energy applications, especially for space heating, timber seasoning and agriculture drying. It has been observed by studying the previous literature that all the elements of a solar air heater such as; an absorber tray, the ducts, glazing, insulation, extended surfaces, as well as the tilt angle, have a significant effect on the thermal performance of the system. This review article focus on the developments that has followed round the globe in various aspects of solar air heating systems since 1877 up to now, with a glimpse of some novel patents of SAHs. The various methods that are used to improve the thermal performance of SAHs such as; optimizing the dimensions of the air heater construction elements, use of extended surfaces with different shapes and dimensions, use of sensible or latent storage media, use of concentrators to augment the available solar radiation, integrating photovoltaic elements with the heaters, etc, are also reported. Besides this, some benefits by using the SAHs has been discussed.

Thermodynamics of micro- and nano-systems driven by periodic temperature variations

Abstract: Heat engines translate thermal energy into useful mechanical work. New results show how the power and efficiency of miniaturized heat engines are related, which paves the way for studies of even smaller systems that experience quantum effects.

A review on desiccant based evaporative cooling systems

Abstract: The air conditioner should control the building sensible and latent load properly in order to provide the indoor comfort conditions. The conventional mechanical vapor compression system usually controls the latent load by the process of condensation of water vapor in which air is cooled below its dew point temperature and then reheated again up to the required supply conditions. The conditions where latent load is dominant these two processes i.e. overcooling and then reheating again will increase the consumption of electrical energy and emission of CO 2 remarkably. To avoid this wastage of primary energy and emission of harmful gases, desiccant based evaporative cooling system is a good alternative to traditional air conditioning system which is cost effective as well as environment friendly. It can be driven by thermal energy which makes a good use of solar energy which is free as well as clean. In this paper, a review of desiccant based evaporative cooling systems has been presented. The present study is undertaken from variety of aspects including background and need of alternative cooling systems, concept of conventional and desiccant based evaporative coolers, system configurations, operational modes, as well as current status of the desiccant based evaporative cooling technology. The review work indicated that the technology of desiccant based evaporative cooler has a great potential of providing human thermal comfort conditions in hot and humid climatic conditions at the expense of less primary resources of energy as compared to conventional cooling systems. Some modified and modern evaporative coolers have also been introduced in this paper.

Broadband absorption engineering of hyperbolic metafilm patterns

Abstract: Perfect absorbers are important optical/thermal components required by a variety of applications, including photon/thermal-harvesting, thermal energy recycling, and vacuum heat liberation. While there is great interest in achieving highly absorptive materials exhibiting large broadband absorption using optically thick, micro-structured materials, it is still challenging to realize ultra-compact subwavelength absorber for on-chip optical/thermal energy applications. Here we report the experimental realization of an on-chip broadband super absorber structure based on hyperbolic metamaterial waveguide taper array with strong and tunable absorption profile from near-infrared to mid-infrared spectral region. The ability to efficiently produce broadband, highly confined and localized optical fields on a chip is expected to create new regimes of optical/thermal physics, which holds promise for impacting a broad range of energy technologies ranging from photovoltaics, to thin-film thermal absorbers/emitters, to optical-chemical energy harvesting.

Heating and cooling energy trends and drivers in buildings

Abstract: The purpose of this paper is to provide a source of information on thermal energy use in buildings, its drivers, and their past, present and future trends on a global and regional basis. Energy use in buildings forms a large part of global and regional energy demand. The importance of heating and cooling in total building energy use is very diverse with this share varying between 18% and 73%. Biomass is still far the dominant fuel when a global picture is considered; the role of electricity is substantially growing, and the direct use of coal is disappearing from this sector, largely replaced by electricity and natural gas in the most developed regions. This paper identifies the different drivers of heating and cooling energy demand, and decomposes this energy demand into key drivers based on a Kaya identity approach: number of households, persons per household, floor space per capita and specific energy consumption for residential heating and cooling; and GDP, floor space per GDP, and specific energy consumption for commercial buildings. This paper also reviews the trends in the development of these drivers for the present, future – and for which data were available, for the past – in 11 world regions as well as globally. Results show that in a business-as-usual scenario, total residential heating and cooling energy use is expected to more or less stagnate, or slightly decrease, in the developed parts of the world. In contrast, commercial heating and cooling energy use will grow in each world region. Finally, the results show that per capita total final residential building energy use has been stagnating in the vast majority of world regions for the past three decades, despite the very significant increases in energy service levels in each of these regions.

Phase change materials for solar thermal energy storage in residential buildings in cold climate

Abstract: Heating accounts for a large proportion of energy consumption in residential buildings located in cold climate. Solar energy plays an important role in responding to the growing demand of energy as well as dealing with pressing climate change and air pollution issues. Solar energy is featured with low-density and intermittency, therefore an appropriate storage method is required. This paper reports a critical review of existing studies on thermal storage systems that employ various methods. Latent heat storage using phase change materials (PCMs) is one of the most effective methods to store thermal energy, and it can significantly reduce area for solar collector. During the application of PCM, the solid–liquid phase change can be used to store a large quantity of energy where the selection of the PCM is most critical. A numerical study is presented in this study to explore the effectiveness of NH 4 Al(SO 4 ) 2 ·12H 2 O as a new inorganic phase change material (PCM). Its characteristics and heat transfer patterns were studied by means of both experiment and simulation. The results show that heat absorption and storage are more efficient when temperature of heat source is 26.5 °C greater than the phase transition temperature. According to heat transfer characteristics at both radial and axial directions, it is suggested to set up some small exchangers so that solar energy can be stored unit by unit in practice. Such system is more effective in low density residential buildings.

Heat-to-current conversion of low-grade heat from a thermocapacitive cycle by supercapacitors

Abstract: Thermal energy is abundantly available, and especially low-grade heat is often wasted in industrial processes as a by-product. Tapping into this vast energy reservoir with cost-attractive technologies may become a key element for the transition to an energy-sustainable economy and society. We propose a novel heat-to-current converter which is based on the temperature dependence of the cell voltage of charged supercapacitors. Using a commercially available supercapacitor, we observed a thermal cell-voltage rise of around 0.6 mV K−1 over a temperature window of 0 °C to 65 °C. Within our theoretical model, this can be used to operate a Stirling-like charge–voltage cycle whose efficiency is competitive to the most-efficient thermoelectric (Seebeck) engines. Our proposed heat-to-current converter is built from cheap materials, contains no moving parts, and could operate with a plethora of electrolytes which can be chosen for optimal performance at specific working temperatures. Therefore, this heat-to-current converter is interesting for small-scale, domestic, and industrial applications.

Worldwide overview of solar thermal cooling technologies

Abstract: In this paper, a brief overview of different available and actually installed solar thermal driven technologies used for cooling or air-conditioning purposes have been presented. A review analysis has been performed taking into account research on experimental and simulated solar cooling systems in terms of COP, area of collector (Ac) per unit chiller capacity (Pch) and volume of storage tank (V) per unit area of collector. The COP of absorption chillers lies between 0.6 and 0.8 for simulated and 0.40–0.85 for experimental systems for generator inlet temperature between 70 and 100 °C with dominance in the market. Adsorption chillers have lower COP in the range of 0.2–0.6 both for simulated and experimental systems. However, adsorption chillers can work at lower generator inlet temperatures in the range of 45–65 °C. The ratio of Ac to Pch presents a wide spread ranging from 1.5 to 11.5 m2 kWch−1. It shows that some installation may have additional area of collector installed resulting in higher thermal energy losses and initial cost of the system while others may have lower Ac installed resulting in lower solar fraction and lesser primary energy savings. Similarly, the ratio of V to Ac also shows a large variation in both simulated and experimental systems ranging from 5 to 130 l m−2. However, most of these systems lie in the range of 20–80 l m−2. The range of storage volume is suggested between 50 and 110 l m−2 but at a first glance it seems that keeping fixed storage volume linked to area of collector may not be an economical solution for large collector areas. Although these installations are designed for solar cooling/air-conditioning purposes only but utilization of these installations further for space heating and sanitary hot water production so called multi-purpose solar thermal systems (MPSTS) will provide better results of primary energy savings.

Heat utilisation technologies: A critical review of heat pipes

Abstract: In electrical or thermal appliances, heat (thermal energy) must either be added into or removed from a system to maintain operational stability. Heat pipes can enhance the heat transfer capabilities without needing a significant temperature gradient between heat sources and heat sinks. The effectiveness of heat pipes is due to the latent heat of phase change of the working fluid within (i) condensation and (ii) evaporation stages. The latent heat of phase change greatly exceeds the sensible heat capacity. Heat pipes may rely on gravity, wicks, centrifugal force or in some cases even a magnetic field to help return condensate flow from the condenser to the evaporator. Wicks in heat pipes are classified into three groups: sintered, groove and mesh types. This review attempts to cover various types of heat pipes such as thermal diodes, variable conductance, pulsating, etc. The application of nanotechnology in heat pipes can be separated into two groups: nanoparticles and nanobubbles, with the latter receiving considerably less attention than the former. The hybridisation of heat pipe technology is also possible and has been discussed along with its future research potential.

Low-Concentration Solar-Power Systems Based on Organic Rankine Cycles for Distributed-Scale Applications: Overview and Further Developments

Abstract: This paper is concerned with the emergence and development of low- to medium-grade thermal-energy conversion systems for distributed power generation based on thermodynamic vapour-phase heat-engine cycles undergone by organic working-fluids, namely organic Rankine cycles (ORCs). ORC power systems are, to some extent, a relatively established and mature technology that is well-suited to converting low-/medium-grade heat (at temperatures up to ~ 300 – 400 °C) to useful work, at an output power scale from a few kW to 10s of MW. Thermal efficiencies in excess of 25% are achievable at higher temperatures and larger scales, and efforts are currently in progress to improve the overall economic viability, and thus uptake, of ORC power systems by focusing on advanced architectures, working-fluid selection, heat exchangers and expansion machines. Solar-power systems based on ORC technology have a significant potential to be used for distributed power generation, by converting thermal energy from simple and low-cost non-concentrated or low-concentration collectors to mechanical, hydraulic or electrical energy. Current fields of use include mainly geothermal and biomass/biogas, as well as the recovery and conversion of waste heat, leading to improved energy efficiency, primary energy (i.e. fuel) use and emission minimization, yet the technology is highly transferable to solar power generation as an affordable alternative to small- to medium-scale photovoltaic (PV) systems. Solar-ORC systems offer naturally the advantages of providing a simultaneous thermal-energy output for hot water provision and/or space heating, and the particularly interesting possibility of relatively straightforward on-site (thermal) energy storage. Key performance characteristics are presented, and important heat transfer effects that act to limit performance are identified as noteworthy directions of future research for the further development of this technology.