• Energy Research

The optimal design of distributed generation systems is of foremost importance to reduce fossil fuel consumption and mitigate the environmental impact of human activities in urban areas. Moreover, an efficient and integrated control strategy is needed for each of the components of a distributed generation plant, in order to reach the expected economic and environmental performances. In this paper, the transition from natural gas to electricity-based heating is evaluated for residential applications, considering the interplay between photovoltaic electricity produced on site and the thermal energy storage, to grant the optimal management of heating devices. The energy demand of an apartment building, under different climatic conditions, is taken as a reference and four power plant solutions are assessed in terms of energy cost and pollution reduction potential, compared to a baseline plant configuration. The performance of each power plant is analyzed assuming an optimized control strategy, which is determined through a graph-based methodology that was previously developed and validated by the authors. Outcomes from our study show that, if heat pumps are used instead of natural gas boilers, energy costs can be reduced up to 41%, while CO2 emissions can be reduced up to 73%, depending on the climatic conditions. Our results provide a sound basis for considering the larger penetration of photovoltaic plants as an effective solution towards cleaner and more efficient heating technologies for civil applications. The simultaneous utilization of heat pumps (as substitutes of boilers) and photovoltaic panels yields a positive synergy that nullifies the local pollution, drastically cuts the CO2 emission, and guarantees the economical sustainability of the investment in renewable energy sources without subsidiary mechanisms.

Thermoelectric generators (TEGs) are solid-state devices that convert temperature gradients into electrical energy. TEGs are desirable sources of energy as they require low maintenance and have high reliability with potential for use in low power and remote applications. To date, no simple and affordable, common test platform for evaluating the performance of TEGs in their associated environment exists. This project is aimed at providing such a platform to deliver performance data that is crucial in power budgeting and viability studies of TEG-powered applications. The design utilizes the common LTC3109 converter chip and an off-the-shelf micro-controller for temperature and load current measurements. The setup automatically regulates the electronic load to achieve maximum power transfer and measurement data can either be observed in real-time through a USB-serial host communication port or logged at a user adjustable rate.

A two-phase thermosyphon is a passive system utilizing gravity to transfer working fluids. The working fluids of a two-phase thermosyphon must undergo vaporization and condensation in the same system. Two-phase thermosyphons can also be used as solar collectors. Traditional solar collectors utilize surface absorbers to convert incident solar radiation into thermal energy, but those systems feature a large temperature difference between the surface absorbers and heat transfer fluids, resulting in a reduction in the overall thermal efficiency. Volumetric solar absorbing fluids serve both as solar absorbers and heat transfer fluids, therefore significantly improving the overall efficiency of solar collectors. Comparing to pure fluids, nanofluids possess both enhanced thermal conductivity and solar absorption capacity as volumetric absorbing fluids. Nanofluids, when serving as volumetric solar absorbing fluids, are so far reported to work only at relatively low temperatures and in a single-phase heat transfer regime due to stability issue. This research investigates the possibility of using nanofluids, especially graphene oxide (GO) nanofluids, as volumetric solar absorbing fluids in two-phase thermosyphons. Despite their reputation as both stable and solar absorptive among nanofluids, graphene oxide nanofluids still deteriorate quickly under boiling-condensation processes (~100 °C). The solar transmittance of the GO nanofluids declines from 38 to 4%, during the first 24 h of testing. Further investigation shows that the stability deterioration is caused by the thermal reduction of GO nanoparticles, which mainly featured with de-carboxylation and de-hydroxylation. A commercial dye named acid black 52, when dissolved in water, exhibits great broadband solar absorption properties and excellent stability. It remains stable for over 199 days in two-phase thermosyphon, and their transmittance in solar spectral region varies less than 9%. The stability of acid black 52 aqueous solution is further confirmed with the 191-day enhanced radiation test, as it shows less than 5% transmittance change in solar spectral region.

This study evaluates the use of international thermal comfort standards currently being used in the south terminal of the King Abdulaziz International Airport (KAIA). The airport is located in Jeddah, in the Kingdom of Saudi Arabia (KSA). The study was prompted by the hypothesis that the hot climate experienced in KSA, the acclimatization of those in the region, coupled with the widespread wearing of traditional clothing, justifies a unique model of thermal comfort. The south terminal of KAIA currently uses set points of 20-24 °C (ASHRAE-based comfort model). Most international thermal comfort standards are based on experiments that were conducted in moderate climates. These studies have two particular shortcomings in the context of those living in hot climates: they fail to consider that people in other climatic regions could have different thermal expectations and preferences, and many disregard the role of outdoor temperature on thermal comfort. The international standards prescribe temperature set points that are often too low for people who live in extremely hot and humid climates. Keeping the temperature at the international set point requires excessive amounts of energy and is wasteful and expensive. Public policy demands that the thermal control strategies in public buildings be evaluated to ensure that they are operating efficiently. Airports are of particular concern because they have HVAC systems that consume a disproportionate amount of energy relative to their size. This study, based on passenger surveys and energy simulation, considers the effectiveness of developing a model of thermal comfort as an alternative control strategy for the KAIA terminal and assesses its energy impact. In order to determine new temperature set points that might better serve the needs of the passengers and maximize energy efficiency in KAIA, this study: a) Conducted detailed surveys of passengers in the airport terminal; b) Obtained measurements of both physical and personal variables; c) Recorded behavior patterns of passengers; d) Collected all relevant data on the conditions inside and out of the terminal; e) Considered the impact that traditional garments may have on thermal comfort; f) Used the data from the surveys to create a new model of thermal comfort; g) Used computer simulation programs to test and compare a developed thermal comfort model with the set point currently used in the building. The results of the survey demonstrate the unsuitability of the ASHRAE-based comfort model (set temperatures of 20-24 °C) currently used in the airport. The data from the survey is used to derive new models of thermal comfort using regression analysis. Computer simulation demonstrated that the new set comfort temperatures obtained from created models could significantly increase the operational efficiency of the terminal. Implementing these models would also reduce the operating cost of the KAIA, lower the CO2 emissions and improve the comfort of passengers. More particularly, the results of the research demonstrate the unsuitability of employing generic comfort models and suggest that a more climate-appropriate strategy should be adopted globally. The Gulf Region Countries do not currently have climatic-specific thermal comfort standards nor intensive field studies that would support their development. Moreover, a vast majority of thermal comfort research is focused on Australia, Europe and the USA and some areas of Asia. This thesis offers an integrated system and methodological approach to evaluate, measure, and analyze both environmental and personal variables of thermal comfort as well as verifying the results and virtually testing the implications on occupants and the building. The objective of carrying out such a study is based on the challenge of achieving acceptable levels of occupant thermal comfort while optimizing the energy efficiency of buildings.

CIES2020: As Energias Renováveis na Transição Energética: Livro de Comunicações do XVII Congresso Ibérico e XIII Congresso Ibero-americano de Energia Solar. Helder Gonçalves, Manuel Romero (Ed.). Lisboa, Portugal: LNEG, 3-5 Novembro, 2020, p. 131-136. ISBN: 978-989-675-076-3

Thermal conductivity plays an important role in energy storage when the materials are charging and discharging. This paper presents an experimental investigation of the evolution of thermal conductivity up to 600 °C in different concretes. Moreover, the thermal conductivity was measured during thermal fatigue cycles when temperature ranged between 300 and 600 °C, simulating the operation conditions in a storage system of molten salts in a Concentrating Solar Power Plant (CSP). Five concrete compositions were analysed using diverse types of aggregates with different thermal response, covering a wide range of the initial thermal conductivity. The results confirm that the loss of thermal conductivity with temperature during the first heating is mainly due to the free water loss. Moreover, the type of aggregate influences the overall thermal performance of concrete due to its thermal conductivity and the volumetric differences with the cement paste. Siliceous aggregates underwent the highest decrease of thermal conductivity of concrete (+50%) with regard to room temperature. Regarding the cooling phase, thermal conductivity recovers between 20% and 40% depending on the type of aggregate. The outcomes of the present study demonstrate that the assumption of a constant thermal conductivity value in numerical simulations to predict its thermal capacity for energy storage is not appropriate. Es una corrección a un artículo publicado en: Sol. Energy 214 (2021) 430–442. Peer reviewed

[EN]The expected performance of an innovative Pumped Thermal Energy Storage (PTES) system based on a closedloop Brayton-Joule cycle and integrated with a Concentrated Solar Power (CSP) plant are analysed in this study. The integrated PTES–CSP plant includes five machines (two compressors and three turbines), a central receiver tower system, three water coolers and three Thermal Energy Storage (TES) tanks, while argon and granite pebbles are chosen as working fluid and storage media, respectively. A sizing of the main components of the integrated plant has been firstly carried out for the design of an integrated PTES-CSP plant with a nominal net power of 5 MW and a nominal storage capacity of 6 equivalent hours of operation. Specific mathematical models have been developed in MATLAB-Simulink to simulate the PTES and CSP subsystem in different operating conditions, and to evaluate the thermocline profile evolution within the three storage tanks during/charging and discharging processes. A control strategy has finally been developed to determine the operating modes of the plant based on the grid service request, the solar availability, and the TES levels. The performance of the system during a summer and a winter day have been analysed considering the integration of the PTES subsystem in the Italian energy market for arbitrage. Results have demonstrated the technical feasibility of the hybridization of a PTES system with a CSP plant and the ability of the integrated system to participate to energy arbitrage, although a lower exergy roundtrip efficiency (about 54 %) has been observed with respect to the sole PTES system (about 60 %).