Showing papers on "Thermal efficiency published in 2014"

Solar steam generation by heat localization

Abstract: Currently, steam generation using solar energy is based on heating bulk liquid to high temperatures. This approach requires either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum. New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these losses. Here we report development of an approach and corresponding material structure for solar steam generation while maintaining low optical concentration and keeping the bulk liquid at low temperature with no vacuum. We achieve solar thermal efficiency up to 85% at only 10 kW m(-2). This high performance results from four structure characteristics: absorbing in the solar spectrum, thermally insulating, hydrophilic and interconnected pores. The structure concentrates thermal energy and fluid flow where needed for phase change and minimizes dissipated energy. This new structure provides a novel approach to harvesting solar energy for a broad range of phase-change applications.

Review of High-Temperature Central Receiver Designs for Concentrating Solar Power

Abstract: This paper reviews central receiver designs for concentrating solar power applications with high-temperature power cycles Desired features include low-cost and durable materials that can withstand high concentration ratios (~1000 suns), heat-transfer fluids that can withstand temperatures >650 °C, high solar absorptance, and low radiative and convective heat losses leading to a thermal efficiency >90% Different receiver designs are categorized and evaluated in this paper: (1) gas receivers, (2) liquid receivers, and (3) solid particle receivers For each design, the following information is provided: general principle and review of previous modeling and testing activities, expected outlet temperature and thermal efficiency, benefits, perceived challenges, and research needs Emerging receiver designs that can enable higher thermal-to-electric efficiencies (50% or higher) using advanced power cycles such as supercritical CO 2 closed-loop Brayton cycles include direct heating of CO 2 in tubular receiver designs (external or cavity) that can withstand high internal fluid pressures (~20 MPa) and temperatures (~700 °C) Indirect heating of other fluids and materials that can be stored at high temperatures such as advanced molten salts, liquid metals, or solid particles are also being pursued, but challenges include stability, heat loss, and the need for high-temperature heat exchangers

High-efficiency thermodynamic power cycles for concentrated solar power systems

Abstract: This paper provides a review of high-efficiency thermodynamic cycles and their applicability to concentrating solar power systems, primarily focusing on high-efficiency single and combined cycles. Novel approaches to power generation proposed in the literature are also highlighted. The review is followed by analyses of promising candidates, including regenerated He-Brayton, regenerated CO2-Brayton, CO2 recompression Brayton, steam Rankine, and CO2–ORC combined cycle. Steam Rankine is shown to offer higher thermal efficiencies at temperatures up to about 600 °C but requires a change in materials for components above this temperature. Above this temperature, CO2 recompression Brayton cycles are shown to have very high thermal efficiency, potentially even exceeding 60% at 30 MPa maximum pressure and above 1000 °C maximum temperature with wet cooling. An estimate of a combined receiver and power cycle operating temperature is provided for the cycles considered and compared to the traditional approach of optimization based on the Carnot efficiency. It is shown that the traditional approach to optimizing the receiver and turbine inlet temperatures based on Carnot is generally not sufficient, leading to an optimum temperature shift of more than 100 °C from the Carnot case under various conditions.

Hybrid pressure retarded osmosis-membrane distillation system for power generation from low-grade heat: thermodynamic analysis and energy efficiency.

Abstract: We present a novel hybrid membrane system that operates as a heat engine capable of utilizing low-grade thermal energy, which is not readily recoverable with existing technologies. The closed-loop system combines membrane distillation (MD), which generates concentrated and pure water streams by thermal separation, and pressure retarded osmosis (PRO), which converts the energy of mixing to electricity by a hydro-turbine. The PRO-MD system was modeled by coupling the mass and energy flows between the thermal separation (MD) and power generation (PRO) stages for heat source temperatures ranging from 40 to 80 °C and working concentrations of 1.0, 2.0, and 4.0 mol/kg NaCl. The factors controlling the energy efficiency of the heat engine were evaluated for both limited and unlimited mass and heat transfer kinetics in the thermal separation stage. In both cases, the relative flow rate between the MD permeate (distillate) and feed streams is identified as an important operation parameter. There is an optimal rela...

A review of hydrogen and natural gas addition in diesel HCCI engines

Abstract: Homogeneous charge compression ignition (HCCI) engine uses a relatively new mode of combustion technology. In principle, there is no spark plug or injector to assist the combustion, and the combustion auto-ignites in multiple spots once the mixture has reached its chemical activation energy. The challenges in developing HCCI engines are the difficulties in: controlling the auto-ignition of the mixture and the heat release rate at high load operations, achieving a cold start, meeting emission standards and controlling knock. At low engine speeds, early auto-ignition can occur, possibly leading to knocking, while late auto-ignition at high engine speeds will make HCCI susceptible to misfire. Hydrogen greatly reduces emissions levels but with reduced power. However, when hydrogen is combined with diesel in dual-fuel mode, low NOx, CO and particulate matter (PM) emissions levels can be achieved, and engine efficiency can be increased by 13–16%. Numerical methods are commonly used to predict HCCI engines' performance (i.e. emissions levels, brake thermal efficiency and combustion phasing), which is cost-effective compared to solely relying on experimentation. The multi-zone method promises better simulation results compared to the single-zone model by combining detailed chemical kinetics with simplified 3D modeling so that turbulence and inhomogeneity in the mixture are considered; good agreement between simulations and experiments have been achieved. Specific strategies used in the experimental method (e.g. fuel additives, inlet air heating, inlet air pressurizing, exhaust gas recirculation (EGR) and injection methods), and numerical method (e.g. single-zone and multi-zone models, mixing model, turbulence model and multi-dimensional model), and other issues associated with HCCI engines are discussed in this paper.

Hybrid microwave-cavity heat engine.

Abstract: We propose and analyze the use of hybrid microwave cavities as quantum heat engines. A possible realization consists of two macroscopically separated quantum-dot conductors coupled capacitively to the fundamental mode of a microwave cavity. We demonstrate that an electrical current can be induced in one conductor through cavity-mediated processes by heating up the other conductor. The heat engine can reach Carnot efficiency with optimal conversion of heat to work. When the system delivers the maximum power, the efficiency can be a large fraction of the Carnot efficiency. The heat engine functions even with moderate electronic relaxation and dephasing in the quantum dots. We provide detailed estimates for the electrical current and output power using realistic parameters.