Computational Fluid Dynamics Modeling of Mixed Convection Flows in Building Enclosures

In recent years Computational Fluid Dynamics (CFD) simulations are increasingly used to model the air circulation and temperature environment inside the rooms of residential and office buildings to gain insight into the relative energy consumptions of various HVAC systems for cooling/heating for climate control and thermal comfort. This requires accurate simulation of turbulent flow and heat transfer for various types of ventilation systems using the Reynolds-Averaged Navier-Stokes (RANS) equations of fluid dynamics. Large Eddy Simulation (LES) or Direct Numerical Simulation (DNS) of Navier-Stokes equations is computationally intensive and expensive for simulations of this kind. As a result, vast majority of CFD simulations employ RANS equations in conjunction with a turbulence model. In order to assess the modeling requirements (mesh, numerical algorithm, turbulence model etc.) for accurate simulations, it is critical to validate the calculations against the experimental data. For this purpose, we use three well known benchmark validation cases, one for natural convection in 2D closed vertical cavity, second for forced convection in a 2D rectangular cavity and the third for mixed convection in a 2D square cavity. The simulations are performed on a number of meshes of different density using a number of turbulence models. It is found that k-epsilon two-equation turbulence model with a second-order algorithm on a reasonable mesh gives the best results. This information is then used to determine the modeling requirements (mesh, numerical algorithm, turbulence model etc.) for flows in 3D enclosures with different ventilation systems. In particular two cases are considered for which the experimental data is available. These cases are (1) air flow and heat transfer in a naturally ventilated room and (2) airflow and temperature distribution in an atrium. Good agreement with the experimental data and computations of other investigators is obtained.

Corrosion of Austenitic Alloys in Binary 60/40 Nitrate Salt at 600°C

Industrial power utilities are using molten binary nitrate salt as a heat transfer fluid and thermal storage media for solar energy generation. Currently, the maximum bulk temperature is 565°C, due to concerns of salt degradation and materials compatibility with containment vessels. To increase overall cycle efficiency, one must increase the upper temperature of the nitrate salt, thereby lowering the levelized cost of electricity (LCoE) through higher power cycle efficiency. The corrosion performance of 316 stainless steel and Inconel 625 is currently characterized at 600°C. However, the 316SS has exhibited stress corrosion cracking (thought due to aqueous flush in Solar Two [1]), and while In625 performs well, its cost is prohibitive. Therefore, current research seeks to evaluate heat-resistant austenitic alloys for use with nitrate salts, ascertaining if they have superior performance characteristics, as well as assessing their mechanisms of corrosion. Sandia National Laboratory is researching four alloys (S35140, ATI332Mo, RA330, and HA556) for corrosion performance at 600°C for 3000 hours, under a cover gas of air. Air is used to simulate the chemistry conditions expected in a power plant. This work details the corrosion rate and the oxide structure for each alloy. Research indicates all alloys are very corrosion-resistant, with metal loss rates projected to be less than 21μm/year after 3000 hours. Though all alloys performed well, corrosion rate data for RA330 (Fe-19Cr-35Ni + minor elements) currently appears to exhibit a linear loss mechanism. In conclusion, this paper will explore the differences in oxide formation between these similar alloys.

Investigation of Hydrodynamics and Heat Transfer Effects due to Light Guides in a Column Photobioreactor

Proper light penetration is an essential design consideration for effective algae growth in column photobioreactors. This research focuses on the placement of light guides within a photobioreactor (PBR), and the effect they have on heat transfer, mass transfer, bubble and fluid flow patterns, and mixing. Studies have been done on a rectangular column photobioreactor (34.29 cm long × 15.25 cm wide × 34.29 cm tall) with two light panels along the front and back of the PBR. A bubble sparger is placed along the center of the bottom length of the PBR with both height and width of 1.27 cm and a length of 33.02 cm. Different configurations and numbers of light guides (1.27 cm diameter) running horizontally from the front to the back of the PBR are modeled using the Computational Fluid Dynamics (CFD) software Star-CCM+. It is hypothesized that the addition of light guides will change the flow pattern but not adversely affect the heat or mass transfer of the carbon dioxide bubbles within the PBR. Potential concerns of light guide placement include inhibiting the flow of the carbon dioxide bubbles or creating regions of high temperature, which could potentially kill the algae. Benefits of light guides include increased light penetration and photosynthesis within the PBR. Five different light guide setups are tested with the carbon dioxide bubbles and water modeled as a turbulent multiphase gas-liquid mixture. The near wall standard k-epsilon two layer turbulence model was used, as it takes into account the viscosity influences between the liquid and gaseous phases. Eight different bubble volumetric flow rates are simulated. The bubble flow patterns, temperature distribution, Nusselt number, Reynolds number, and velocity are all analyzed. The results indicate square arrays of light guides give the most desirable velocity distribution, with less area of zero velocity compared to the staggered light guide setup. Temperature distribution is generally even for all configurations of light guides.

Thermodynamic Analysis of a Central Heating System Combing the Urban Heat Network With Geothermal Energy

In recent years, with the continuous urban expansion, the central heating sources are commonly insufficient in the areas of Northern China. Besides, the increasing heat transfer temperature difference results in more and more exergy loss between the primary heat network and the secondary heat network. This paper introduces a new central heating system which combines the urban heat network with geothermal energy (CHSCHNGE). In this system, the absorption heat exchange unit, which is composed of an absorption heat pump and a water to water heat exchanger, is as alternative to the conventional water to water heat exchanger at the heat exchange station, and the doing work ability of the primary heat network is utilized to drive the absorption heat pump to extract the shallow geothermal energy. In this way, the heat supply ability of the system will be increased with fewer additional energy consumptions. Since the water after driving the absorption heat pump has high temperature, it can continue to heat the supply water coming from the absorption heat pump. As a result, the water of the primary heat network will be stepped cooled and the exergy loss will be reduced. In this study, the performance of the system is simulated based on the mathematical models of the heat source, the absorption heat exchange unit, the ground heat exchanger and the room. The thermodynamic analyses are performed for three systems and the energy efficiency and exergy efficiency are compared. The results show that (a) the COP of the absorption heat exchange unit is 1.25 and the heating capacity of the system increases by 25%, which can effectively reduce the requirements of central heating sources; (b) the PER of the system increases 14.4% more than that of the conventional co-generation central heating system and 54.1% more than that of the ground source heat pump system; (c) the exergy efficiency of the CHSCHNGE is 17.6% higher than that of the conventional co-generation central heating system and 45.6% higher than that of the ground source heat pump system.

Investigation of Thermochemical Hydrogen Production via the Novel Thermo-Mechanical Stabilized Iron Oxide-Zirconia Porous Structure

In this study we have developed a unique method for synthesizing very reactive water splitting materials that will remain stable at temperatures as high as 1450 °C to efficiently produce clean hydrogen from concentrated solar energy. The hydrogen production for a laboratory scale reactor using a “Thermo-mechanical Stabilized Porous Structure” (TSPS) is experimentally investigated for oxidation and thermal reduction temperatures of 1200 and 1450 °C, respectively. The stability and reactivity of a 10 g TSPS over many consecutive oxidation and thermal reduction cycles for different particle size ranges has been investigated. The novel thermo-mechanical stabilization exploits sintering and controls the geometry of the matrix of particles inside the structure in a favorable manner so that the chemical reactivity of the structure remains intact. The experimental results demonstrate that this structure yields peak hydrogen production rates of 1–2 cm3/(min.gFe3O4) during the oxidation step at 1200 °C and the 30 minute thermal reduction step at 1450 ° C without noticeable degradation over many consecutive cycles. The hydrogen production rate is one of the highest yet reported in the open literature for thermochemical looping processes using thermal reduction. This novel process has strong potential for developing an enabling technology for efficient and commercially viable solar fuel production.

Waste From Metallurgic Industry: A Sustainable High-Temperature Thermal Energy Storage Material for Concentrated Solar Power

The ambitious DOE SunShot cost target ($0.06/kWh) for concentrated solar power (CSP) requires innovative concepts in the collector, receiver, and power cycle subsystems, as well as in thermal energy storage (TES). For the TES, one innovative approach is to recycle waste from metallurgic industry, called slags, as low-cost high-temperature thermal energy storage material. The slags are all the non-metallic parts of cast iron which naturally rises up by lower density at the surface of the fusion in the furnace. Once cooled down some ceramic can be obtained mainly composed of oxides of calcium, silicon, iron, and aluminum. These ceramics are widely available in USA, about 120 sites in 32 States and are sold at a very low average price of $5.37/ton. The US production of iron and steel slag was estimated at 19.7 million tons in 2003 which guarantees a huge availability of material. In this paper, electric arc furnace (EAF) slags from steelmaking industry, also called “black slags”, were characterized in the range of temperatures of concentrated solar power. The raw material is thermo-chemically stable up to 1100 °C and presents a low cost per unit thermal energy stored ($0.21/kWht for ΔT = 100 °C) and a suitable heat capacity per unit volume of material (63 kWht/m3for ΔT = 100°C). These properties should enable the development of new TES systems that could achieve the TES targets of the SunShot (temperature above 600 °C, installed cost below $15/kWht, and heat capacity ≥25 kWht/m3). The detailed experimental results are presented in the paper. After its characterization, the material has been shaped in form of plates and thermally cycled in a TES system using hot-air as heat transfer fluid. Several cycles of charge and discharged were performed successfully and the concept was validated at laboratory scale. Apart from availability, low-cost, and promising thermal properties, the use of slag promotes the conservation of natural resources and is a noble solution to decrease the cost and to develop sustainable TES systems.

Design and Construction of a Solar Thermal Refrigeration System for Patna, India

This paper describes the design and construction of a solar thermal adsorption refrigerator in Patna, Bihar, India. After a brief description of the local situation and planning prerequisites the reasons for choosing an ethanol based adsorption system are explained. The following sections are focused on the description of the design and the theory behind the system. Lastly, practical aspects that arose during the construction of the first prototype are mentioned.

Analysis of a Hybrid PV/T Concept Based on Wavelength Selective Films

The technical performance of a non-tracking hybrid PV/T concept that uses a wavelength selective film is modeled. The wavelength selective film is coupled with a compound parabolic concentrator to reflect and concentrate the infrared portion of the solar spectrum onto a tubular absorber while transmitting the visible portion of the spectrum to an underlying thin-film photovoltaic module. The optical performance of the CPC/selective film is obtained through Monte Carlo Ray-Tracing. The CPC geometry is optimized for maximum total energy generation for a roof-top application. Applied to a rooftop in Phoenix, Arizona USA, the hybrid PV/T provides 20% more energy compared to a system of the same area with independent solar thermal and PV modules, but the increase is achieved at the expense of a decrease in the electrical efficiency from 8.8% to 5.8%.

Design Point for Predicting Year-Round Performance of Solar Parabolic Trough Concentrator Systems

Thermal efficiencies of the solar field of two different parabolic trough concentrator (PTC) systems are evaluated for a variety of operating conditions and geographical locations, using a detailed 3D heat transfer model. Results calculated at specific design points are compared to yearly average efficiencies determined using measured direct normal solar irradiance (DNI) data as well as an empirical correlation for DNI. It is shown that the most common choices of operating conditions at which solar field performance is evaluated, such as the equinox or the summer solstice, are inadequate for predicting the yearly average efficiency of the solar field. For a specific system and location, the different design point efficiencies vary significantly and differ by as much as 11.5% from the actual yearly average values. An alternative simple method is presented of determining a representative operating condition for solar fields through weighted averages of the incident solar radiation. For all tested PTC systems and locations, the efficiency of the solar field at the representative operating condition lies within 0.3% of the yearly average efficiency. Thus, with this procedure, it is possible to accurately predict year-round performance of PTC systems using a single design point, while saving computational effort. The importance of the design point is illustrated by an optimization study of the absorber tube diameter, where different choices of operating conditions result in different predicted optimum absorber diameters.

Transient Three-Dimensional Heat Transfer Model of a Solar Thermochemical Reactor for H2O and CO2 Splitting via Nonstoichiometric Ceria Redox Cycling

A transient heat transfer model is developed for a solar reactor prototype for H2O and CO2 splitting via two-step non-stoichiometric ceria cycling. Counter-rotating cylinders of reactive and inert materials cycling between high and low temperature zones permit continuous operation and heat recovery. To guide the reactor design a transient three-dimensional heat transfer model is developed based on transient energy conservation, accounting for conduction, convection, radiation, and chemical reactions. The model domain includes the rotating cylinders, a solar receiver cavity, and insulated reactor body. Radiative heat transfer is analyzed using a combination of the Monte Carlo method, Rosseland diffusion approximation, and the net radiation method. Quasi-steady state distributions of temperatures, heat fluxes, and the non-stoichiometric coefficient are reported. Ceria cycles between temperatures of 1708 K and 1376 K. A heat recovery effectiveness of 28% and solar-to-fuel efficiency of 5.2% are predicted for an unoptimized reactor design.