Derek K. Baker

Designing Solar Hot Water Systems for Scaling Environments

Component failures and system performance degradation in SHW systems due to scaling are common in areas with hard water. It appears that many valve and pump failures on the potable water side are related to scaling, and any scale build-up on heat transfer surfaces will result in performance degradation. Different designs are compared in regard to their susceptibility to problematic scaling. Indirect systems utilizing external and tank wall heat exchangers are compared in regard to the rate of scaling and the consequences of scaling on system performance. The tank wall heat exchanger appears preferable over a doubly pumped external heat exchanger, both in terms of system reliability and resistance to performance degradation.

Predicted Impact of Collector and Zeolite Choice on the Thermodynamic and Economic Performance of a Solar-Powered Adsorption Cooling System

Transient thermodynamic and capital cost models for an intermittent solar-powered adsorption cooling system are presented. The models are used to predict size and cost trends as the type of zeolite and collector, and maximum zeolite temperature (T Z,Max), are varied. The best synthetic and natural zeolites considered have similar performance. The minimum cost system uses a flat plate collector for T Z,Max < 160°C and an evacuated tube collector for T Z,Max > 160°C. Collector costs dominate the system costs. In some cases, the zeolites adsorption characteristics are predicted to exert a larger influence on system cost than the actual cost of the zeolite.

Identifying and Reducing Scaling Problems in Solar Hot Water Systems

In areas with hard water, scaling can reduce the reliability of solar hot water (SHW) systems. Common reliability problems associated with scaling are both mechanical (collector freeze damage, clogged passages, premature failure of pumps and valves) and thermal (efficiency degradation). A mechanistic and a mathematical scaling rate model are presented. Results from controlled experiments investigating the effect of key water chemistry and heat transfer parameters on the scaling rate are summarized. The implications of these results for designing SHW systems for scaling environments are discussed. Most importantly, indirect systems where the potable water side of the heat exchanger is integrated into the storage tank wall, such as in a wrap-around heat exchanger, are shown to be the most mechanically and thermally reliable systems for scaling environments. A new version of the software SolScale is discussed, which is intended to aid in the design of SHW systems to reduce scaling related reliability problems.Copyright

Technical Study of a Hybrid Solar–Geothermal Power Plant and Its Application to a Thermal Design Course

An energetic model for a hybrid solar–geothermal electric power plant (HSGEPP) is developed to explore the extent to which solar thermal resources can extend and enhance marginal and declining geothermal fields. The model is developed and presented to allow replication in a 4th-year solar engineering thermal design course. The model is applied to a HSGEPP being developed in Turkey, and simulations are run using a typical meteorological year formatted data set. The solar fraction ( f s) of the HSGEPP is equal to the fractional decrease in the geothermal resource usage. The increase in annual f s with the collector field’s solar multiple (Ms) is linear up to approximately fs = 0.25 for Ms = 1.25, after which the rate of increase in fs begins to decay and fs approaches 0.37 for Ms = 5. For Ms = 1.25, the monthly solar fraction ranges from 0.05 in December to 0.43 in July.

Trends in COP for Adsorption Cooling Cycles With Thermal Regeneration and Finite Number of Beds

A thermodynamic model is developed to predict trends in limiting COP of an adsorption cooling cycle with thermal regeneration between n beds, where n is any even number and each bed is spatially isothermal. The results of the model indicate the optimum distribution of beds throughout the cycle to maximize thermal regeneration. Simulations were run for silica gel-water and zeolite-water adsorbent-refrigerant pairs as the maximum bed temperature and the bed’s sensible load were varied. For the silica gel-water pair, the exothermic adsorption process occurs at lower temperatures than the endothermic desorption process, which prevents the latent loads from being thermally regenerated. This inability to regenerate latent loads results in a relatively small opportunity to increase COP through thermal regeneration, and this opportunity decreases rapidly with increasing number of beds. Conversely, for the zeolite-water pair much of the exothermic adsorption process occurs over the same temperature range as the endothermic desorption process, which allows a significant portion of the latent loads to be thermally regenerated. This ability to regenerate latent loads results in a much larger opportunity to increase COP through thermal regeneration, and this opportunity decreases much more gradually with increasing number of beds.Copyright

Numerical Analysis of Phase Change Material Characteristics Used in a Thermal Energy Storage Device

ABSTRACT In this study, a numerical analysis is performed to investigate the freezing process of phase change materials (PCM) in a predesigned thermal energy storage (TES) device. This TES device is integrated with a milk storage cooling cycle operating under predefined practical conditions. Using this cooling unit, 100 litres of milk is kept cool at 4°C for 48 hours before it is collected. A 2-D model of the TES device is developed in COMSOL Multiphysics to analyze the phase change performance of water-based PCMs. The variations of thermal properties with temperature during the phase change are considered in the analysis. The model is used for exploring the solidification process of PCMs inside the TES device. Temperature variations with time, ice formation, and the impacts of boundary conditions are investigated in detail. Water PCM shows better characteristics in the solidification process in comparison to eutectic PCMs, which is mainly due to the differences between phase change temperatures of the PCMs.

Experimental Investigation of Single-Phase Liquid Flow and Heat Transfer in Multiport Minichannels

Small channels have been an area of interest since the 1970s owing to their enhanced heat transfer characteristics. However a wide number of studies in literature show inconsistent results. In this work, an experimental set-up has been designed and constructed to investigate pressure drop and heat transfer characteristics of single-phase water flow in rectangular multiport minichannels. Laminar flow inside three minichannels with different hydraulic diameters and different port numbers were examined under a constant heat flux boundary condition. The results are presented in terms of Poiseuille number (Po) and average Nusselt number (Nu). Generally, average Nusselt number results and Poiseuille number results showed good agreement with constant Po theory and constant Nu theory, excluding developing effects and experimental errors. On the other hand, developing effects are found to be increasing as hydraulic diameter decreases. Similarly, constant Nu value showed a decrease with increasing hydraulic diameter. The experimental results are compared with conventional correlations. While the agreement with conventional correlations is satisfactory, the predictions of the correlations overestimated most of the results. No early transition was observed for Reynolds number (Re) smaller than 1800.Copyright

Proposal of a novel gravity-fed, particle-filled solar receiver

Solar Thermal Electricity power plants utilizing solid particles as heat transfer and storage media have been proposed by several research groups, with studies citing benefits of increased thermal efficiency and lower cost. Several types of solid particle receivers have been proposed, with leading designs consisting of particles falling or suspended in air. A new solid particle receiver is proposed here, consisting of a receiver fully packed with particles flowing downward with gravity. Particle flow rate is regulated with an outlet valve. This Particle-Filled receiver concept is compared to other receiver designs, and initial cold and hot experiments are conducted. Mass flux values of up to 379 kg m−2 s−1 are demonstrated, and heat transfer coefficients between 136 and 251 W m−2 K−1 are found.Solar Thermal Electricity power plants utilizing solid particles as heat transfer and storage media have been proposed by several research groups, with studies citing benefits of increased thermal efficiency and lower cost. Several types of solid particle receivers have been proposed, with leading designs consisting of particles falling or suspended in air. A new solid particle receiver is proposed here, consisting of a receiver fully packed with particles flowing downward with gravity. Particle flow rate is regulated with an outlet valve. This Particle-Filled receiver concept is compared to other receiver designs, and initial cold and hot experiments are conducted. Mass flux values of up to 379 kg m−2 s−1 are demonstrated, and heat transfer coefficients between 136 and 251 W m−2 K−1 are found.

Numerical Comparison and Sizing of Sensible and Latent Thermal Energy Storage for Compressed Air Energy Storage Systems

Compressed Air Energy Storage is a promising large-scale storage system in part because of its high power rating during discharge. But it is not the cleanest way of storing energy due to the necessity of an external heat source (typically the combustion of natural gas) to heat the air at the turbine inlet. This problem can be overcome with Thermal Energy Storage by storing the thermal energy of air at the compressor exhaust in order to be used for heating air before turbine. In this study, a numerical transient heat transfer model of Thermal Energy Storage is developed and the performance of Thermal Energy Storage is investigated based on heat storage capacity, required time to store unit amount of energy and air temperature profiles at the outlet of Thermal Energy Storage during discharge for the system. High heat storage per volume is necessary for more compact systems. Required time to store unit amount of energy is desired to be short for a fixed volume Thermal Energy Storage in order to maintain continuous operation; on the other hand, air at the outlet (turbine inlet) should be at a high temperature for the longest time possible to supply hot air to turbine. In order to investigate the effects of operating parameters, different volumes of Thermal Energy Storage tank filled with different storage mediums of various sizes are explored. Latent Heat and Sensible Heat Thermal Energy Storage systems are compared using magnesium chloride hexahydrate, paraffin, myristic acid and naphthalene as phase change materials and rock as sensible storage medium. Results show that Latent Heat Thermal Energy Storage gives a better performance than Sensible Heat Thermal Energy Storage. Among phase change materials, magnesium chloride hexahydrate provides the highest heat storage per volume. Required time to store unit amount of energy are comparable among the phase change materials. Magnesium chloride hexahydrate seems promising considering the discharge temperature profile at the Thermal Energy Storage outlet. Capsule size should be kept as small as possible which can be challenging in terms of manufacturing.Copyright

Technical Analysis of a Hybrid Desiccant Cooling in a Mediterranean Climate

This chapter presents a performance analysis of a hybrid liquid desiccant cooling system (DCS) with lithium chloride as the desiccant material in a Mediterranean climate. Mathematical models of desiccant contactors are adopted from the literature. The complete system is modeled in the TRNSYS platform and is simulated using Typical Meteorological Year data. The building model was developed in accordance with the building’s construction material and usage style. Simulations are performed over the summer period of a typical year and the results of the system are compared with the results of a conventional vapor compression cooling system (VCCS) from viewpoint of system characteristics and energy savings. One of the important contributions is the conclusion that a transient cycle analysis is necessary to understand the DCS cycle’s performance. The results also indicate that the consumed energy in both systems is approximately equal in magnitudes but different in type; DCS shifts the energy required for the operation of the system from electrical energy to thermal energy. It is also observed that for large supply-airflow rate applications, DCS would be more beneficial than VCCS.