Open Rack Research Articles

Floating photovoltaics: modelled and experimental operating temperatures and the impact of wind speed and direction

Floating photovoltaics (FPV) is rapidly emerging as a promising alternative to ground-mounted PV (GPV) where available land area is scarce or expensive. Improved cooling has often been reported as a benefit of FPV, as cell temperature is an important parameter for the performance of a PV system. However, more recent literature shows that the cooling effect depends strongly on FPV technology and that it is not always superior to that of open rack GPV systems. There is still a need for more information on how to estimate cell temperatures for FPV systems, and how to consider the influence of various environmental factors such as wind speed and direction. Operating cell temperature may be estimated with the PVsyst model, where heat loss coefficients (U-values) denote the heat transfer capabilities of the PV system. In this work, cell temperatures and U-values for a small footprint FPV system with east-west orientation and a 15° tilt located in Sri Lanka are studied using both module temperature measurements and computational fluid dynamics (CFD) modelling. CFD modelling allows for investigating the influence of both wind speed and direction on cell temperatures, as well as to look at the distribution of cell temperatures within the system under different wind conditions. Calculations based on measurements give Uc = 22.6 W/m2K and Uv = 4.9 Ws/m3K and correlate well with CFD calculations. We also show that wind direction, system configuration and sensor placement influence the estimated U-values, complicating the use of tabulated values for any given technology.

Assessing the combined effects of local climate and mounting configuration on the electrical and thermal performance of photovoltaic systems. Application to the greater Sydney area

Extremely high urban temperatures adversely affect photovoltaic (PV) system performance. Accurate PV cell temperature assessment relies on local weather conditions, exacerbated by urban overheating, often overlooked by inadequate temperature models and non-local data. This study investigates the electrical and thermal PV performance, considering mounting configurations and local conditions. Data from ten weather stations in Greater Sydney (NSW) during 2016–2017, including a hot summer, are used. The Sandia model is used to predict cell temperatures and power output for four mounting configurations, from open rack to building-integrated (BIPV). A PV thermal model is implemented to analyse daytime convection, crucial for understanding PV impact on local climate. Results show peak cell temperatures of 60 °C (open rack) to over 90 °C (BIPV), causing up to 50% power loss and 11% reduction in monthly performance ratio. Local climate variations impact PV energy output up to 6%, with mounting configuration effects up to 11%. Daytime convective flux averages 150–180 W/m2, peaking at 700 W/m2. Convective release varies up to 22% based on local climate, generally higher for open rack than close roof mounts, with potential reversals under low wind speed conditions. These findings can improve the knowledge of PV performances in urban areas facing extreme temperatures.

EFFECTS OF BULB SIZE, DEPTH OF STORAGE AND BULB ORIENTATION ON STORAGE LOSS OF ONION (Allium cepa L.)

Onion is a major crop grown in northern Nigeria. It is popular for its nutritional and medicinal benefits worldwide. However, a lot of postharvest losses occurs especially during the storage of onion. This work was carried out to investigate the effects of bulb size, depth of storage and bulb orientation on physiological weight loss, sprouting and rotting of onion (red creole variety) in the modified IAR open rack onion storage structure for a sampling period of three months. A Complete Randomized Design (CRD) Anova was employed. Data were collected for a sampling period of 12 weeks and analysed using the Statistical Analysis Software (SAS) at 1% and 5% significant levels. Duncan Multiple Range Test (DMRT) was used for the mean separation of the significant variables. The results obtained showed that the total weight loss of onions during the sampling period was 19.6%. Bulb orientation had a major effect on sprouting and physiological weight loss. Medium bulb sizes of (5 – 7cm Major Diameter) were better suited for storage.

Development of a physics-based surrogate model using two-dimensional first principle equations and optimization of open rack vaporizer

In an effort to provide stable and affordable city gas, the open rack vaporizer (ORV), a heat exchanger that vaporizes Liquefied Natural Gas (LNG) using seawater, is receiving significant attention. In this study, a physics-based surrogate model for the ORV system with high fidelity and low computational load compared to a computational fluid dynamics (CFD) model is developed based on two-dimensional (2D) first principle equations of energy and mass balances for heat transfer. Partial key parameters are estimated using the mean squared error based parameter subset selection method with global sensitivity analysis to address the overfitting issue. The resulting physics-based surrogate model shows robust performance and approximates the temperature profile with an error of less than 5% compared to the CFD data. Using the surrogate model, operating conditions are optimized with both multi-objective and single-objective functions. The results indicate that the two objectives, profit and LNG outlet temperature, are incompatible and that two operating conditions, LNG and seawater flowrates, are inversely proportional.

Mission profile concept for PV modules: use case – middle east deserts vs temperate European climate

The paper addresses the need to simplify the comparison between different climatic conditions. Therefore, the concept of Mission Profiles is introduced to PV. Mission Profiles are a set of climatic stresses compiled to allow direct comparisons such as temperature, humidity, and UV exposure. The use-case in this paper is a comparison of a temperate with a hot climate. The results show that temperature driven processes are highly accelerated for the hot climate, whereas processes that involve humidity may lead to a similar annual degradation. The impact of light, particularly UV, is even more complex. UV triggered reaction processes can be photon absorption triggered and accelerated by temperature. However, for most observed interactions and material changes UV light goes in conjunction with moisture (H2O) for the chemical reaction and is accelerated by temperature. Calculating the impact of temperature and UV is possible. However, the impact of moisture is almost impossible as it requires the knowledge of the local microclimate. Only a “worst-case” scenario can be assumed while using ambient relative humidity for most events of the day, that, however, do typically not include condensation. With the introduction of Mission Profiles, it should be possible to assess stress more easily for particular sites in general. Differences in mounting such as open rack compared to e.g., BiPV can also better addressed, however, with some assumption to define the microclimate (e.g., exclude rear side cooling).

무선 통신 기반 해수식 기화기 운영 상태 모니터링 시스템 개발

An open rack vaporizer is a facility that vaporizes liquefied natural gas using sea water. When a vaporization efficiency of the open rack vaporizer decreases, liquefied natural gas can leak, which can cause great damage to the facility. Operators have to monitor the operation state of the facility in real-time to prevent the accident. However, operators have visited the site and have checked the state by looking at the value of sensors installed in the open rack vaporizer through indicators. For the safe operation of the open rack vaporizer, a monitoring system is needed to monitor the operation state of the open rack vaporizer in real-time without the need for operators to visit the site. In this paper, we developed a long term evolution based monitoring system to monitor the operation state of the open rack vaporizer. The developed system can monitor the real-time operation state of the open rack vaporizer at a control center far from the facility. For the system development, data transmission infrastructure using long term evolution was built. Afterwards a software was developed to monitor the operation state of the open rack vaporizer in real-time using the transmitted data. Finally, performance evaluation was conducted to confirm that the developed system operated successfully without data transmission delay or data missing.

A Comparative Study of Energy Savings in a Liquid-Cooled Server by Dynamic Control of Coolant Flow Rate at Server Level

Data center proliferation has been increasing significantly around the world attributed to growth in technologies such as the Internet of Things (IoT), bitcoin mining, and high-performance computing (HPC). A direct consequence of these developments is an enhancement in processing units and a corresponding rise in CPU and GPU power densities. Limitations of air cooling to dissipate increasing power densities in processors have compelled the researchers to move toward better and efficient liquid cooling solutions. In an earlier study, a custom-made mini-rack with liquid-cooled 2OU (open rack Unit) web servers were tested for comparison of centralized and distributed pumping with constant flow rates at the server level. The effect of higher inlet temperature in terms of IT power, cooling power consumption, and CPU temperature was reported along with a comparison of centralized versus distributed pumping. In this article, the same 2OU server is used to show the effect of variable flow rate on server thermal performance. The parameters monitored for performance quantification are the core temperatures, dual in-line memory modules (DIMM) temperatures, and platform controller hub (PCH) temperature. These parameters were reported by varying the CPU power consumption, coolant inlet temperatures, and coolant flow rates by controlling the distributed pumps. The server was fully enclosed with no outside air intake where the CPUs were cooled by cold plates and the rest of the components with internally recirculating air. The results obtained from the experiments were compared with the results obtained from a previous study where the effect of the variable flow rate was ignored. A full-factorial design of experiments (DoE) was designed using Minitab 19 to analyze the results statistically.

The potential of cold energy extraction from a power plant regasification terminal for cold storage application

Cold storage is a chamber used for food preservation. The food is stored inside the cold storage at a specific temperature and relative humidity to slow down the deterioration process caused by pathogenic organisms. The type of food stored requires specific thermal conditions e.g 0-5 °C with RH of 80-90% for vegetables, 4-5 °C for milk processing, and -25 to -45 °C for quick freezing of fish. Cold energy from the LNG regasification process is a promising option for cold storage application. Normally, prior to supply to industrial users e.g power plants, the LNG is regasified by dissipating its cold energy to another working fluid of vapourizer e.g seawater in the Open Rack Vapourizer (ORV) or ambient air in the Ambient Air Vapourizer (AAV). Recovering this cold energy to cool the cold storage can significantly reduce the energy consumption in the cooling process. Simultaneously, it reduces greenhouse gas emissions. This study aims to investigate the potential of wasted LNG cold energy in a power plant’s regasification terminal, located in Aceh, as a source of energy to cool cold storage. The cooling load of cold storage under four different capacity namely 5, 10, 15, and 20 tons of fish is calculated and compared with the potential cold energy available. This study then concludes the cold storage capacity that meets the available LNG cold energy.

Standards development for modules in high temperature micro‐environments

AbstractPhotovoltaic (PV) module qualification standards, IEC 61215 and IEC 61730, were designed to apply to “general open‐air climates” and IEC 61730 specifically indicated applicability of ambient air temperature of 40°C. Additionally, these standards provided allowances for so‐called “open rack mounted PV modules” without a clear definition of “open‐rack.” These implied restrictions and allowances meant that hotter climates or thermally restrictive installation methods may not be covered by these often customer‐mandated certification standards. This is particularly salient for the significant growth regions of the Middle East and India that would be expected to operate at significantly higher temperatures. The applicability of these documents raised issues over the definition of “open rack” and the fact that the geographic location is just as important as the mounting configuration in assessing the impact of the micro‐environment of a PV module. This work summarizes the scientific background for IEC Technical Specification 63126:2020 ED1, titled “Guidelines for qualifying PV modules components and materials for operation at high temperatures.” This standard was recently published by the IEC and is the first step in a systematic effort to rework these standards to address the question of temperature more directly. Instead of specifying a mounting condition, we specify different suites of tests suitable for a system (PV module, mounting style, and location) defined by the 98th percentile cell temperature. With a defined temperature regime to work from, this allowed us to use existing literature research combined with additional modeling work to determine, which tests would need to be modified. This resulted in suggested changes to material thermal indices, thermal cycling temperatures, hot spot testing, ultraviolet testing, and bypass diode testing among other tests and characteristics described herein.

Lean lubrication of ultra large modulus open gear and rack pair: A case study of the gear-rack drive mechanism of the Chinese “Three Gorge Dam” ship lift

Large modulus gear-rack mechanism has strong bearing capacity. They are often used in heavy-duty equipment, i.e. for heavy load lifting. This study is focused on the ultra large modulus heavy duty open gear-rack drive mechanism of the Chinese “Three Gorge Dam” ship lift, of which lubrication design and management plays a critical role in safe operation and service life extension of the mechanism. Against the currently adopted manual oiling which raises concerns from ineffective lubrication, high lubricate wastes as well as risks of maintainers, this study aims to propose lean lubrication design method based on parametric modeling and numerical analysis. A three-dimensional linear contact hybrid elastohydrodynamic lubrication model was established to characterize lubrication behavior under effects of contact area size and morphology. With the EHL model, lean oil supply conditions were exploited based on effective lubricating oil thickness. Given the uncertainty and complexity of lubrication state in actual working conditions, texture design was then proposed for better tribological performance under starved lubrication conditions. The proposed models and methods were applied to the gear-rack drive of the Chinese “Three Gorge Dam” ship lift. The numerical results suggested the proposed lean lubrication plan may consume 4.13% less lubricating oil in one mesh cycle than that calculated by conventional methods. The reduction of lubricating oil consumption can be larger if the high waste from oil oversupply by manual oiling is considered. The output of case study may serve as the basis of further development of lean lubrication system for the gear-rack drive mechanism.

Single Objective Optimization of Net Zero Energy Residential Building in Tropical Climate

The Building sector is the largest consumer of energy worldwide. Net Zero Energy Residential Building (NZERB) is one of the prudent approaches to reduce pressure on primary sources of energy. The main objective of this study is to use Taguchi simulation based on orthogonal arrays approach to optimize thermal envelope of a building, SEER value of HVAC system, lighting power density, and ventilation level to reduce the Building Energy Intensity (BEI) 50% lower than the base case building according to International Energy Conservation Codes 2015. In this study whole building energy simulation was performed by Hourly Analysis Energy Program and PVWatts calculator was used to estimate the size of the PV system for balancing the energy demand of the house. The results of Taguchi simulation approach show that by decreasing the U value of wall, windows, roof, floor to 0.1, 0.8, 0.1, 0.2 W/m2. °K respectively and ventilation rate 0.15 l/s-m2, lights of 110 lm/W and SEER of the HVAC system 4 (W/W option) reduce BEI from 147.78 kWh/m2/year for base case building to 67.085 kWh/m2/year for optimal design case. PV system size estimation was performed with PVWatts calculator. PV Watts recommended the size of 13kW DC rating system (open rack type) of 15% efficiency for the weather of Johor Bahru to meet the annual energy demand of the proposed optimal building design.

Effects of drying technique on the quality of Mola (Amblypharyngodon mola) dried by solar tent dryer and open sun rack dryer

Quality change of Mola (Amblypharyngodon mola) from open sun rack and solar tent dryer was evaluated at 1st, 2nd and 3rd days of drying. Sensory analysis showed that, solar tent dryer dry fish were excellent in overall quality of dry fish from open sun rack dryer. Moisture content of Mola from solar tent dryer was significantly (p>0.05) lower than the moisture content of dry fish from open sun rack dryer. Protein, lipid, ash and TVB-N content was significantly (p>0.05) higher in solar tent dried fish than dried fish from open sun rack. However, microbial load was higher on dry fish obtained from open sun rack dryer compared with the dry fish from low coat solar tent dryer. Moreover, the quality of the dried fish products from solar tent dryer was excellent in quality compared to the dried fish from open sun rack dryer.
 Res. Agric., Livest. Fish.7(1): 121-128, April 2020

Implicit regression-based correlations to predict the back temperature of PV modules in the arid region of south Algeria

The determination of the PV module temperature is a key parameter for the assessment of the actual performance of the PV systems. The application of available models for PV module temperature estimation in literature can be verified, but the application of these correlations for different climate conditions does not lead to unequivocal results. The main objective of this study is to suggest new empirical models for estimating the back surface module temperature under outdoor hot dry climatic conditions of Adrar province (Algerian Sahara) and to compare the developed models to different existing models in the literature. The models are developed based on meteorological and irradiance data collected from two different plants with different module technologies. The best site-specific approach uses a simple formula to derive the PV-back module temperature from the meteorological variables such as ambient temperature, and irradiance. The relative root mean square error and the Pearson’s correlation coefficient of the best developed model are 10.662% and 0.955, respectively. In addition, MAPE and RMSE values are considerably small for the studied stations. A general model for predicting the PV-back temperature was also recommended for simple PV modules or open rack systems in rural locations with no measurement equipment nearby. The results are quite useful for studying PV system performance and estimating its energy output.

Experimental study on heat transfer characteristics of cooling falling film outside a vertical tube in open rack vaporizer

The falling film flow has gained wide attention in recent years, due to its extraordinary performance in different heat exchangers in terms of high heat transfer coefficient and low energy consumption. In this paper, a series of experiments are conducted in a smooth tube and a finned tube respectively, in order to investigate the heat transfer characteristics of the cooling falling film flow outside vertical tubes in open rack vaporizer (ORV). Different control variables affecting the heat transfer characteristics are studied, including the mass flow rate and the temperature of water, the heat flux and the tube shape. The experimental data are compared with the predicted values from existing correlations, and all of them show large discrepancies. Therefore, a new correlation is proposed to accurately predict the heat transfer coefficient of the falling film flow based on the experimental data. The predicted values of the new correlation are within an error band of 10% of the experimental data. The study may provide reference for the research on the falling film flow and effective design and operation of ORV.

Experimental investigation on heat transfer characteristics of supercritical nitrogen in a heated vertical tube

The rapid variations of physical properties near the pseudocritical points and the unique heat transfer and resistance characteristics of supercritical fluids have gained a great deal of attentions in recent years. In this paper, the heat transfer characteristics of supercritical nitrogen in a heated vertical tube are investigated experimentally. The test section is an aluminum alloy tube of 6.6 m in length and 0.02 m in inner diameter with a wall thickness of 0.0085 m. The effects of different operating parameters, including pressure and mass flow rate of nitrogen, inlet temperature and mass flow rate of water, on the heat transfer characteristics are investigated. Meanwhile, the experimental data are compared with the predict values by different existing correlations, and all of them show large differences. Therefore, a new correlation has been developed to accurately predict the heat transfer coefficient based on the experimental data. The new correlation works well with 95% of the experimental data captured with a deviation of 20%. The experimental study and the new correlation may provide reference for the study of the heat transfer of supercritical fluid flow and design and operation of open rack vaporizer (ORV).

Cold energy recovery performance study for liquefied natural gas (LNG) regasification process

The Earth's cleanest burning fossil fuel - natural gas (NG), is primarily methane (CH4) with smaller quantities of other hydrocarbons. As the grid is decarbonizing, the role of NG on the road to a decarbonized future is indisputable. NG is often compressed and converted into liquified natural gas (LNG) that occupies 600 times less space than its gaseous form, optimizing its storability and delivery efficiency. This logistical flexibility helps improve the security of NG supplies worldwide and is making LNG one of the fastest-growing energy markets. At the LNG terminals, LNG is converted back into its gaseous state by regasification, then, distributed across the network, from remote production areas to distant markets where NG supplies are needed. Regasification of LNG releases a significant amount of cold energy. This paper examines the effect of Open Rack Vaporizer (ORV) efficiency and the potential of recoverable cold energy in the LNG regasification process. All data were collected from the industry-leading LNG regasification terminal in Malaysia - The Pengerang LNG (Two) SDN. Bhd. (PLNG2). The results of this study showed that the monthly potential of recoverable cold energy is around 43 MW and the efficiency of ORV is ranged from 60 % to 95 %. In addition, it has been found that the flow rate of LNG and the flow rate of seawater are two significant parameters that affect the potential recoverable cold energy.

Virtual Pipeline System

After LNG vessel arrives at receiving port, the cargo will unload to the storage tank first. When natural gas delivery is requested, it will transport through the pipeline network to customers’ sites. Prior pumping in the pipeline, the liquefied natural gas (LNG) will go through vaporization process to re-gasify LNG into gaseous form. The popular LNG vaporizer systems include ambient air vaporizer, open rack vaporizer (using sea water), submerged combustion vaporizer and intermediate fluid vaporizer. The regasification process will occupy spaces and external energy will be used, more or less. The SOFC – Solid Oxide Fuel Cell takes hydrocarbon fuel as feedstock, such as natural gas (methane), and goes through a series of chemical process (not burning), then generates electricity with high conversion efficiency plus some high quality heat. Using the heat for LNG re-gasification and forming a close-loop energy supply system become very attractive in terms of compactness, economy and easy to operate. Especially for those remote communities, rural areas and remote islands, the energy access is usually not available. The combination of SOFC and re-gasification module will perfect the downstream operation in natural gas value chain.

Experimental and simulation investigation on heat transfer characteristics of supercritical nitrogen in a new rib tube of open rack vaporizer

Open rack vaporizer (ORV), using seawater as heat source, is widely used in liquefied natural gas (LNG) receiving terminals. In this study, a new rib tube, with petal-shaped internal fins and rectangular straight external fins, is designed and tested. A series experiments are conducted to investigate the heat transfer and pressure drop characteristics of the supercritical nitrogen flow in the new rib tube and the smooth tube. The heat transfer coefficient of the rib tube is 66.3% larger than that of the smooth tube, and the frictional pressure drop in the rib tube is also smaller compared to that of the smooth tube. Different control variables affecting the heat transfer characteristics in the rib tube are studied, including the temperature of the water, the mass flow rate and the pressure of the nitrogen. Meanwhile, numerical simulation is conducted to further analyze the heat transfer coefficients near the pseudo-critical region. A new correlation is developed to accurately predict the heat transfer coefficients of the supercritical fluid in the new rib tube based on the experimental and simulation data. The new correlation shows a good agreement as 95% of the experimental data captured with an error within 20%. This study may provide reference for the research on the supercritical fluid flow and the heat transfer enhancement of the tube in ORV.

Alternative of LNG cold exergy utilization for generating electrical energy at Gresik LNG receiving terminal

Gresik LNG receiving terminal will be built to meet the needs of combined-cycle power plant (PLTGU) with natural gas regasification rate of 60.95 MMSCFD. In conventional regasification process using open rack vaporizer (ORV), the potential of LNG cold exergy will be wasted to seawater which is used as a heat source from the environment. The commercially proven scheme of the regasification process with the utilization of LNG cold exergy to generate electrical energy such as Direct Expansion, Rankine Cycle and combined Direct Expansion+Rankine Cycle is simulated with Unisim computer software. The purpose of this study is to assess technically and economically three scheme mention above. The results show that the combined scheme has the ability to produce the largest electricity surplus of 39.80 kWh per ton LNG regasified with potential revenue from electrical energy sales of USD 1,140,935 per year. The energy and exergy analysis shows that this scheme also has highest thermal efficiency of 14.48% and highest exergy efficiency of 60.71%. However, based on the results of economic analysis found that Direct Expansion scheme has the highest NPV among the three schemes with a value of USD 695,032.

Thermal performance analysis and the operation method with low temperature seawater of super open rack vaporizer for liquefied natural gas

Super open rack vaporizer (SuperORV) is a type of liquefied natural gas (LNG) vaporizer which utilizes the sensible heat of seawater. Because of its compact structure and low operating cost, SuperORV is more and more prevalent among the world’s LNG receiving stations. In this paper, a heat transfer model of a tube of SuperORV is established, and the model is validated by operation data. The profiles of the natural gas (NG) temperature and the heat transfer coefficient along the tube under the design condition are calculated and analyzed. The NG outlet temperature and the length of ice formed on the outside wall of the tube are analyzed to evaluate the thermal performance under off-design conditions. Besides, the operation method of SuperORV with low temperature seawater is discussed. It is found that the reduction of the LNG mass flow rate is an efficient method for the operation of SuperORV with cold seawater. The capacity of the vaporizer with various seawater temperatures is analyzed, and the calculation shows that the SuperORV can work with 2 °C seawater if the LNG mass flow rate is reduced to 46% of the design value.