Thermal management of concentrator photovoltaics

Photovoltaic Concentrator systems, which increase the solar radiation intensity on the photovoltaic cells, may reduce the system cost, if the cost of the concentrator is less than the photovoltaic material displaced. An Asymmetric Compound Parabolic Photovoltaic Concentrator (ACPPVC) for building façade integration with a solar concentration ratio of 2.0 has been designed, fabricated and experimentally characterised. The truncated ACPPVC has acceptance half angles of 0° and 55° and an absorber width of 125mm. Phase Change Materials (PCM) have been integrated to the rear of the PV panel to moderate the temperature rise of the PV and maintain good solar-electrical conversion efficiency. The thermal behaviour of a Fresnel lens PV Concentrator (FPVC) has also been studied in this work. A two-dimensional ray trace technique has been used to predict the optical performance and the angular acceptance of the ACPPVC system. The predicted highest optical efficiency was 88.67% for the ACPPVC-55 system. Extensive indoor experimental characterisation of a number of PV systems was undertaken for a range of incident solar radiation intensities using a highly collimated solar simulator developed specifically for this project. Experimental results showed that the electrical output from the ACPPVC-55 was approximately 1.8 of that of a non-concentrating PV system with similar solar cells area. The electrical conversion efficiency for the ACPPVC-55 system was further increased, when RT27 PCM was incorporated to its rear

Stable Fulde-Ferrell-Larkin-Ovchinnikov pairing states in 2D and 3D optical lattices

We present the study of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing states in the $p$-orbital bands in both two and three-dimensional optical lattices. Due to the quasi one-dimensional band structure which arises from the unidirectional hopping of the orthogonal $p$-orbitals, the pairing phase space is not affected by spin imbalance. Furthermore, interactions build up high dimensional phase coherence which stabilizes the FFLO states in 2D and 3D optical lattices in a large parameter regime in phase diagram. These FFLO phases are stable with imposing the inhomogeneous trapping potential. Their entropies are comparable to those of the normal states at finite temperatures.Comment: 5 page

Quantum magnetism of ultra-cold fermion systems with the symplectic symmetry

We numerically study quantum magnetism of ultra-cold alkali and alkaline-earth fermion systems with large hyperfine spin $F=3/2$, which are characterized by a generic $Sp(N)$ symmetry with N=4. The methods of exact diagonalization (ED) and density-matrix-renormalization-group are employed for the large size one-dimensional (1D) systems, and ED is applied to a two-dimensional (2D) square lattice on small sizes. We focus on the magnetic exchange models in the Mott-insulating state at quarter-filling. Both 1D and 2D systems exhibit rich phase diagrams depending on the ratio between the spin exchanges $J_0$ and $J_2$ in the bond spin singlet and quintet channels, respectively. In 1D, the ground states exhibit a long-range-ordered dimerization with a finite spin gap at $J_0/J_2>1$, and a gapless spin liquid state at $J_0/J_2 \le 1$, respectively. In the former and latter cases, the correlation functions exhibit the two-site and four-site periodicities, respectively. In 2D, various spin correlation functions are calculated up to the size of $4\times 4$. The Neel-type spin correlation dominates at large values of $J_0/J_2$, while a $2\times 2$ plaquette correlation is prominent at small values of this ratio. Between them, a columnar spin-Peierls dimerization correlation peaks. We infer the competitions among the plaquette ordering, the dimer ordering, and the Neel ordering in the 2D system.Comment: 16 page

Analysis of the daylight performance of a glazing system with Parallel Slat Transparent Insulation Material (PS-TIM)

Daylight plays an important role in the energy efficiency and indoor environmental quality of an office building. An innovative façade system where parallel transparent/translucent plastic slats are sandwiched between glass panes to form a Parallel Slat Transparent Insulation Material (PS-TIM) is proposed as a strategy to effectively increase the thermal resistance of window systems, while providing better daylight performance. In this paper, the optical performance (as defined by Bidirectional Scattering Distribution Function) of a double glazed window containing PS-TIM systems with different slat pitches (the distance between neighbouring slats), slat tilt angles, as well as the slat materials (transparent and translucent) was obtained using a ray-tracing technique. Then, the annual daylight performance of a typical office building with various PS-TIM applied under different climatic conditions and at different orientations was investigated using RADIANCE. The simulation results show that PS-TIM with translucent slats offers better daylight performance than conventional double glazing: it can increase the percentage of annual working hours under daylight, where the illuminance lies in the useful range by up to 79%. It also achieves a homogenous distribution of daylight within the internal working space and effectively reduces the possibility of glare. When applying PS- TIM at higher site latitude, smaller slat pitches are required to maximise useful daylight. Optimised PS-TIM geometry is also affected by local prevailing sky conditions

Experimental investigation of evacuated heat pipe solar collector efficiency using phase-change fluid

Performance of a microencapsulated phase-change material (PCM) as a heat-transport medium in an evacuated heat pipe solar collector was evaluated and the results compared with those using water. Collector efficiency was experimentally determined according to the method based on European Standard EN 12975–2:2006. This method proved unsuitable when using an encapsulated PCM suspension. A modified test method was proposed, which was appropriate for predicting solar collector efficiency when using a phase-change fluid. Average solar collection efficiency when using a PCM suspension was higher than that using water

A review of Transparent Insulation Material (TIM) for building energy saving and daylight comfort

Improving the energy efficiency of buildings is a key strategy in responding to climate change and resource challenges associated with the use of fossil fuel derived energy. The characteristics of the building envelope play a decisive role in determining building operation energy. Transparent Insulation Materials (TIMs) add to the strategies that may be used to sustain these improvements: they can reduce heat loss by providing high thermal resistance while effectively transmitting solar energy and contributing to the luminous environment. In this review, key types of TIMs and their characterisation in terms of both thermal and optical behaviours are introduced as well as the benefits that may be realised through their application to buildings. Relatively few studies exist regarding the performance of window systems incorporating TIMs. To provide a clear picture of how to accurately predict the performance of TIM integrated window systems, this paper also explores the literature around window systems incorporating complex interstitial structures, as these share many of the same characteristics as TIMs. The experimental and numerical methods used to evaluate the thermal and optical characteristics of complex window systems are summarised and this body of research provides potential methods for tackling similar questions posed in relation to the performance of window systems with TIMs. Finally, this review introduces a method that permits the prediction of the combined thermal and daylight behaviour of spaces served by TIM integrated window systems. The results from using this methodology show that using TIMs over a conventional window system offers a range of benefits to the occupants of buildings. Thus, this review offers a workflow that may be used to assess and analyse the benefit of applying TIMs for building energy saving and daylight comfort in buildings subjected to varying climate conditions

Thermal management of concentrator photovoltaics

Photovoltaic Concentrator systems, which increase the solar radiation intensity on the photovoltaic cells, may reduce the system cost, if the cost of the concentrator is less than the photovoltaic material displaced. An Asymmetric Compound Parabolic Photovoltaic Concentrator (ACPPVC) for building façade integration with a solar concentration ratio of 2.0 has been designed, fabricated and experimentally characterised. The truncated ACPPVC has acceptance half angles of 0° and 55° and an absorber width of 125mm. Phase Change Materials (PCM) have been integrated to the rear of the PV panel to moderate the temperature rise of the PV and maintain good solar-electrical conversion efficiency. The thermal behaviour of a Fresnel lens PV Concentrator (FPVC) has also been studied in this work. A two-dimensional ray trace technique has been used to predict the optical performance and the angular acceptance of the ACPPVC system. The predicted highest optical efficiency was 88.67% for the ACPPVC-55 system. Extensive indoor experimental characterisation of a number of PV systems was undertaken for a range of incident solar radiation intensities using a highly collimated solar simulator developed specifically for this project. Experimental results showed that the electrical output from the ACPPVC-55 was approximately 1.8 of that of a non-concentrating PV system with similar solar cells area. The electrical conversion efficiency for the ACPPVC-55 system was further increased, when RT27 PCM was incorporated to its rear.EThOS - Electronic Theses Online ServiceUniversity of Warwick (UoW)GBUnited Kingdo

Smart windows—-dynamic control of building energy performance

This paper explores the potential of thermotropic (TT) windows as a means of improving overall building energy performance. Capitalising on their ability to dynamically alter solar and visible light transmittance and reflectance based on window temperature, they have the ability to reduce solar heat gains and subsequently reduce cooling loads when the external conditions exceed those required for occupant comfort. Conversely when the external conditions fall short of those required for comfort, they maintain a degree of optical transparency thus promoting opportunities afforded by passive solar gains. To test their overall effectiveness, thermotropic layers made of varying hydroxypropyl cellulose (HPC) concentrations (2 wt.%, 4 wt.% and 6 wt.%) were firstly synthesised and their optical properties measured. Building performance predictions were subsequently conducted in EnergyPlus for four window inclinations (90°, 60°, 30° and 0° to the horizontal) based on a small office test cell situated in the hot summer Mediterranean climate of Palermo, Italy. Results from annual predictions show that both incident solar radiation and outdoor ambient temperature play a significant role in the transmissivity and reflectivity of the glazing unit. If used as a roof light, a 6 wt.% HPC-based thermotropic window has a dynamic average Solar Heat Gain Coefficient (SHGC) between 0.44 and 0.56, this lower than that of 0.74 for double glazing. Predictions also show that in the specific case tested, the 6 wt.% HPC-based thermotropic window provides an overall annual energy saving of 22% over an equivalent double glazed unit. By maintaining the thermotropic window spectral properties but lowering the associated transition temperature ranges, it was found that the lowest temperature range provided the smallest solar heat gains. Although, this is beneficial during periods where cooling may be needed, in those periods where heating may be required, opportunities gained through passive solar heating are limited. In addition, with lower solar heat gain, there is a possibility that artificial lighting energy demand increases resulting in additional energy consumption

Thermal evaluation of a double glazing façade system with integrated Parallel Slat Transparent Insulation Material (PS-TIM)

Concerns over sustainability in the built environment have resulted in continuous efforts to improve the performance of glazed façade systems and hence indoor comfort and building energy conservation. An innovative façade system where parallel transparent plastic slats are sandwiched between glass panes to form a Parallel Slat Transparent Insulation Material (PS-TIM) is proposed as a strategy to effectively reduce coupled convective and radiative heat transfer between the panes of a double glazed window. This strategy increases the thermal resistance of the façade, while maintaining access to daylight. A numerical investigation of the thermal and optical performance of this façade system is presented. Detailed modelling of the thermal characteristics of a double glazed window containing PS-TIM systems was carried out for different cell aspect ratios (defined by the thickness of window air cavity and slat interval distance), slat thickness and slat properties (conductivities and emissivities) using a validated Computational Fluid Dynamic (CFD) model. The CFD predictions show that: 1) an aspect ratio of 0.35 can provide full suppression of convection; 2) the PS-TIM structure can achieve a 35–46% reduction in thermal conductance compared with the same double glazing in the absence of PS-TIM; 3) material conductivity, thickness and emissivity have a more apparent influence on small cell structures than large cell structures. In addition, a simple analysis of U-value and light transmittance at various solar incidence angles was undertaken. The results provide a better understanding of the benefits of PS-TIM in terms of energy saving and offer suggestions for the improved design of glazing façade systems