Comparison of Two Linear Collectors in Solar Thermal Plants: Parabolic Trough vs Fresnel

Parabolic trough can be considered the state of the art for solar thermal power plants thanks to the almost 30 years experience gained in SEGS and, recently, Nevada Solar One plants in US and Andasol plants in Spain. One of the major issues that limits the wide diffusion of this technology is the high investment cost of the solar field and, particularly, of the solar collector. For this reason, since several years research activity has been trying to develop new solutions with the aim of cost reduction. This work compares commercial Fresnel technology with conventional parabolic trough plant based on synthetic oil as heat transfer fluid at nominal conditions and evaluates yearly average performances. In both technologies, no thermal storage system is considered. In addition, for Fresnel, a Direct Steam Generation (DSG) case is investigated. Performances are calculated by a commercial code, Thermoflex®, with dedicated component to evaluate solar plant. Results will show that, at nominal conditions, Fresnel technology have an optical efficiency of 67% which is lower than 75% of parabolic trough. Calculated net electric efficiency is about 19.25%, while parabolic trough technology achieves 23.6%. In off-design conditions, the gap between Fresnel and parabolic trough increases because the former is significantly affected by high radiation incident angles. The calculated sun-to-electric annual average efficiency for Fresnel plant is 10.2%, consequence of the average optical efficiency of 38.8%, while parabolic trough achieve an overall efficiency of 16%, with an optical one of 52.7%. An additional case with Fresnel collector and synthetic oil outlines differences among investigated cases. Finally, because part of performance difference between PT and Fresnel is simple due to different definitions, additional indexes are introduced in order to make a consistent comparison.

A Model for a Geothermal Well in a Hydraulic Groundwater Flow for Ground Source Heat Pump Systems

Heat pumps are mechanical systems that provide heating to a space in the winter, and cooling in the summer. They are increasingly popular because the same system provides both cooling modes, depending on the direction of the cycle upon which they operate. For proper operation, the heat pump must be connected to a constant temperature thermal reservoir which in traditional systems is the ambient air. In ground source heat pumps however, subterranean ground water is used as the thermal reservoir. To access the subterranean groundwater, “geothermal” wells are drilled into the formation. Water from the building heating or cooling system is circulated through the wells thereby promoting heat exchange between the coolant water and the subterranean formation. The potential for higher efficiency heating and cooling has increased the utilization of ground source heating ventilating and air conditioning systems. In addition, their compatibility with a naturally occurring and stable thermal reservoir has increased their use in the design of sustainable or green buildings and man-made environments. Groundwater flow affects the temperature response of thermal wells due to advection of heat by physical movement of groundwater through the aquifer. Research on this subject is scarce in the geothermal literature. This paper presents the derivation of an analytical solution for thermal dispersion by conduction and advection from hydraulic groundwater flow for a “geothermal” well. This analytical solution is validated against asymptotic analytical solutions. The traditional constant linear heat source solution is dependent on the ground formation thermal properties; the most dominant of which is the thermal conductivity. The results show that as hydraulic groundwater flow increases, the influence of the ground formation thermal conductivity on the temperature response of the well diminishes. The diminishing influence is evident in the Peclet number parameter; a comparison of thermal advection from hydraulic groundwater flow to thermal conduction by molecular diffusion.

Optimal Placement of Horizontal- and Vertical-Axis Wind Turbines in a Wind Farm for Maximum Power Generation Using a Genetic Algorithm

In this paper, we consider the Wind Farm layout optimization problem using a genetic algorithm. Both the Horizontal–Axis Wind Turbines (HAWT) and Vertical-Axis Wind Turbines (VAWT) are considered. The goal of the optimization problem is to optimally place the turbines within the wind farm such that the wake effects are minimized and the power production is maximized. The reasonably accurate modeling of the turbine wake is critical in determination of the optimal layout of the turbines and the power generated. For HAWT, two wake models are considered; both are found to give similar answers. For VAWT, a very simple wake model is employed.

The Potential of Natural Ventilation in Single-Sided Ventilated Apartment to Improve Indoor Thermal Comfort and Air Quality

Cross ventilation is a more effective ventilation strategy in comparison to single-sided ventilation. In the NSW Residential Flat Design Code1 (RFDC) the majority of apartments are required to adopt cross ventilation. However, in the case of studio and one-bedroom apartments, it is acknowledged that single-sided ventilation may prevail. Deep plan studio and one-bedroom apartments may achieve lower amenity of summer thermal comfort and indoor air quality where mechanical ventilation is not provided by air conditioning. Since compliance with the code may allow up to 40% of apartments in a development in Sydney to be single sided, it is important to understand the natural ventilation performance of such apartments. The objective of this paper is to investigate the natural ventilation potential in single-sided ventilated apartments to improve indoor air quality and thermal comfort. This investigation includes simulating various facade treatments involving multiple opening and balcony configurations. Balcony configurations are included in this study because, in Sydney, a balcony is a compulsory architectural element in any apartment building. The study uses computational fluid dynamics (CFD) software to simulate and predict the ventilation performance of each apartment configuration. This study suggests that properly configured balconies and openings can significantly improve indoor ventilation performance for enhanced indoor air quality and thermal comfort, by optimizing the available prevailing wind. However, it is important to note that inappropriately designed fac¸ade treatments also could diminish natural ventilation performance.

Hydrogen Production via the Iron/Iron Oxide Looping Cycle

The iron/iron-oxide looping cycle has the potential to produce high purity hydrogen from coal or natural gas without the need for gas phase separation: Hydrogen is produced from steam oxidation of iron or Wustite yielding primarily Magnetite; Magnetite is then reduced back to iron/Wustite using syngas (CO+H2). A system model has been developed to identify favorable operation conditions and process configurations. Process configurations for three distinct temperature ranges, (i) 500–950 K, (ii) 950–1100 K, and (iii) 1100–1200 K have been developed. The energy content of high temperature syngas from conventional coal gasifiers is sufficient to drive the looping process throughout the temperature range considered. Temperatures around 1000 K are advantageous for both the hydrogen production step and the iron oxide reduction step. Simulations of a large number of subsequent cycles indicate that quasi-steady operation is reached after approximately 5 cycles. Comparison of simulations and experiments indicate that the process is currently limited by chemical kinetics at lower temperatures. Therefore, product recirculation should be used for a scaled-up process to increase reactant residence times while maintaining sufficient fluidization velocity.

Flex House

Flex House is a flexible, modular, pre-fabricated zero energy building that can be mass produced and adapted easily to a variety of site conditions and plan configurations. The key factor shaping the design is central Florida’s hot humid climate and intense solar radiation. Flex house combines the wisdom of vernacular Florida houses with state of the art Zero Energy House technologies (ZEH.) A combined system of photovoltaic panels and solar thermal concentrating panels take advantage of the region’s abundant insolation in providing clean renewable energy for the house. Conservation is achieved with state of the art mechanical systems and innovative liquid desiccant dehumidification technology along with highly efficient lighting and appliances. The hybrid nature of the Flex house allows for both an open and closed system to take advantage of the seasonal temperature variation. Central Florida buildings can conserve energy by allowing natural ventilation to take advantage of passive cooling in the mild months of the year and use a closed system to utilize mechanical cooling when temperatures are too high for passive cooling strategies. The building envelope works equally well throughout the year combining an optimum level of insulation, resistance to air infiltration, transparency for daylight, and flexibility that allows for opening and closing of the house. Flex House is designed with a strong connection between interior spaces and the outdoors with carefully placed fenestration and a movable wall system which enables the house to transform in response to the temperature variations throughout the year. The house also addresses the massive heat gain that occurs through the roof, which can generate temperatures in excess of 140 degrees. Flex House incorporates a parasol-like outer structure that shades the roof, walls and courtyard minimizing heat gain through the building envelope. To be implemented on a large scale, ZEH must be affordable for people earning a moderate income. Site built construction is time consuming and wasteful and results in higher costs. Building homes in a controlled environment can reduce material waste, and construction costs while increasing efficiency. Pre-fabricating Flex House minimizes preparation time, waste and safety concerns and maximizes economy, quality control, efficiency and safety during the construction process. This paper is an account of the design and construction of Flex House, a ZEH for central Florida’s hot humid climate.

A Methodology for Evaluating the Environmental Trade-Offs for Different Travel and Information Communication Technologies (ICT) Options

The impacts of the U.S. transportation and electricity generation sectors include air emissions and water consumption. Information and communication technologies (ICT) such as advanced video teleconferencing have the potential to displace some activities that have historically required transportation. While ICT can reduce environmental impacts compared to transportation options in many cases, there are non-obvious environmental trade-offs associated with replacing transportation with ICT. These tradeoffs are the consequence of many factors, including the particular local electricity mix, meeting duration, number of meeting participants, travel distances, travel modes, motive transport conversion technologies, and transport fuels. Identifying and quantifying these trade-offs is the focus of this research. For this study, a nomenclature and methodology were developed to compare environmental trade-offs associated with transportation and ICT. The nomenclature was designed to facilitate side-by-side comparison of the environmental impacts of travel and ICT and to allow expansion of the nomenclature for future study. The methodology considered a variety of conversion technologies for motive transport including spark-ignition, compression-ignition, fuel cells, and electric motors. Both conventional and developing fuels were considered including gasoline, ethanol, diesel, biodiesel, natural gas, hydrogen, and electricity. Likewise, electricity consumption for ICT included both traditional and developing electricity generation technologies. Carbon dioxide emissions and water consumption for ICT were assessed for comparison with transportation in a case study that demonstrated use of this methodology by considering three distinct scenarios for a particular business meeting: 1. Two meeting attendees travel to the meeting by diesel city bus while two travel in a private vehicle. 2. All four meeting attendees travel by private vehicle powered by compressed natural gas. 3. The four meeting attendees do not travel, but instead meet their clients virtually via ICT. The case study analyzed in this manuscript considers only the water and carbon dioxide impacts, but the nomenclature developed allows future expansion for analysis of other greenhouse gases. The three scenarios revealed that, compared to short travel distances, use of ICT does not always generate fewer carbon dioxide emissions. Depending on the mode of electricity generation, travel proved to be preferable from an emissions standpoint for scenarios in which travel distances were small. However, in cases that required long distances to travel, ICT often allowed businesses and individuals to reduce their environmental impacts, especially if electric power generation in that location utilized large amounts of relatively low-emissions technologies such as hydroelectric dams, wind, solar, and nuclear. Finally, it should be noted that, in addition to comparing ICT and travel impacts, this methodology can be used to calculate the environmental tradeoffs of various transportation options when travel is a necessity.

Renewable Fuel Performance in a Legacy Military Diesel Engine

A new Hydrotreated Vegetable Oil (HVO) from the camelina plant has been processed into a Hydrotreated Renewable Jet (HRJ) fuel. This HRJ fuel was tested in an extensively instrumented legacy military diesel engine along with conventional Navy jet fuel JP-5. Both fuels performed well across the speed-load range of this HMMWV engine. The high cetane value of the HRJ leads to modestly shorter ignition delay. The longer ignition delay of JP-5 delivers shorter overall combustion durations, with associated higher indicated engine torque levels. Both brake torque and brake fuel consumption are better with conventional JP-5 by up to ten percent, due to more ideal combustion characteristics.

Developments in Light Vehicle Life Cycle Analysis With Application to Electric Vehicles

An LCA tool first reported on at the ASME ES conference in 2007 has been expanded and improved as follows: • More than 400 production vehicles from all over the world are now in the data base. • Conventional and renewable liquid and gas fuels are included. • Electric vehicles (EVs) and plug in hybrid electric vehicles (PHEVs) are included along with hybrid electric vehicles (HEVs) and conventional internal combustion engine vehicles. • The tool is now web-based. The LCA tool includes both fuel and vehicle life cycle coefficients in its data base. To illustrate the LCA ranking of vehicles using electricity (EVs, PHEVs, and HEVs) vs. conventional vehicles this paper will report on greenhouse gas emissions, total life cycle energy use along with NOx, SOx and mercury emissions. It will be shown, for example, that EVs are not the cleanest solution contrary to claims of various commentators in the popular press and of EV enthusiasts who do not take the entire life cycle into account.

Thermal and Mechanical Characterization of Contiguous Wall Systems for Energy Efficient Low Cost Housing

The interaction of alternative technologies for low cost housing using a fully integrated finite element thermal and structural model of the system. The such as matrix formulations, or different wall systems can be accomplished using. Multi-layer systems based on composite laminate theory are used as a substitution for both reinforcement and effective thermal barrier of structural walls and roof systems. Textile Reinforced Cement composites (TRCs) as thin sandwich skin elements are considered since they show improved tension capacity and ductility based a well-bonded and well-distributed reinforcement that minimizes the flaw sizes, leading to the increase in overall strength and ductility. A range of innovative materials are used in a transient thermal analysis of the composite wall system. Using both 2-D and 3-D finite element analysis, field data obtained from interior and exterior faces of three model construction systems are simulated for walls and roof members as a function of time. Using the exterior temperature as the imposed boundary condition, the interior temperatures were predicted and compared with the experimentally obtained results. Sensitivity of the model to changes in parameters is studied for various insulating materials.