The final goal of this study is to implement and to test a thermal energy storage (TES) system using different phase change materials (PCM) for solar cooling and refrigeration applications. A high temperature pilot plant able to test different types of TES systems and materials was designed and built at the University of Lleida (Spain). This pilot plant is composed mainly by three parts: heating system, cooling system, and different storage tanks. The pilot plant uses synthetic thermal oil as heat transfer fluid (HTF) and has a working temperature range from 100 ºC to 400 ºC. Two different PCM were selected after a deep study of the requirements of a real solar cooling plant and the available materials in the market, finally d-mannitol with phase change temperature of 167 ºC and hydroquinone which has a melting temperature of 172.2 ºC were used. ; The work is partially funded by the Spanish government (ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2009 SGR 534). Antoni Gil would like to thank the Col·legid'Enginyers Industrials de Catalunya for his research appointment. Eduard Oró would like to thank the University of Lleida for his research fellowship.
A sustainable and environmentally alternative to commonly used air conditioning systems can be the solar cooling system due to the use of renewable and clean energy. Solar absorption systems can be used for greenhouse cooling in areas with high outdoor temperatures and solar radiation levels. These systems take advantage of the simultaneity between the solar energy availability and the greenhouse cooling demand allowing the reduction of conventional electricity and water consumption. This paper presents the results of the application of a solar cooling plant for the climate control of a greenhouse at the University of Bari, Italy. The experimental plant consists of a Mediterranean greenhouse, having a surface of 300 m2, and of a single effect LiBr-H2O absorption chiller fed by evacuated-tube solar collectors. Two different localized systems were chosen for the distribution of cold inside the greenhouse: the first system presents pipes placed centrally on the cultivation vessels; the second consists of pipes in contact with aluminium plates and of a transparent EVA film, used to border an area close to plants. The distribution system of cold with pipes, plate and EVA film provided a slightly higher cooling capacity due to the presence of the plates which increases the ability to dissipate energy.
Producing cost-competitive small and medium-sized solar cooling systems is currently a significant challenge. Due to system complexity, extensive engineering, design and equipment costs; the installation costs of solar thermal cooling systems are prohibitively high. In efforts to overcome these limitations, a novel sorption heat pump module has been developed and directly integrated into a solar thermal collector. The module comprises a fully encapsulated sorption tube containing hygroscopic salt sorbent and water as a refrigerant, sealed under vacuum with no moving parts. A 5.6m2 aperture area outdoor laboratory-scale system of sorption module integrated solar collectors was installed in Stockholm, Sweden and evaluated under constant re-cooling and chilled fluid return temperatures in order to assess collector performance. Measured average solar cooling COP was 0.19 with average cooling powers between 120 and 200 Wm-2 collector aperture area. It was observed that average collector cooling power is constant at daily insolation levels above 3.6 kWhm-2 with the cooling energy produced being proportional to solar insolation. For full evaluation of an integrated sorption collector solar heating and cooling system, under the umbrella of a European Union project for technological innovation, a 180 m2 large-scale demonstration system has been installed in Karlstad, Sweden. Results from the installation commissioned in summer 2014 with non-optimised control strategies showed average electrical COP of 10.6 and average cooling powers between 140 and 250 Wm-2 collector aperture area. Optimisation of control strategies, heat transfer fluid flows through the collectors and electrical COP will be carried out in autumn 2014.
Abstract Following the increasing awareness on the global warming, international governments have set up severe targets on CO2 emission in transportation sector: the overcoming of these limits produces a fine which directly influences the market value of the vehicle. Moreover, concerning the traditional pollutants, they still remain targeted by future EURO6(b-c-d) limits. Charge air cooling, in turbocharged diesel engines, is widely used to increase air density, improve cylinder filling and, definitively, engine volumetric efficiency. This is usually done through a heat exchanger fed by environmental air positioned after the charge air compression: the cooling is strictly related to vehicle speed, dedicated radiator positioning, engine operating point and environmental temperature. All these factors lead to an in-cylinder intake air temperature in the 40-70°C range. A refrigerating unit, featuring an evaporator, suitably placed inside the intake manifold, could provide an additional cooling. Such an option is not so difficult to be implemented considering that a conditioning unit is already present on board for cabin comfort. This unit often is over-designed and frequently under-employed. In this paper an evaporator was placed on the intake line of a turbocharged diesel engine, available on a test bench, and the effects of the under-cooling of the charge air have been experimentally assessed. The evaporator is fed by an air refrigeration unit present on vehicle board for cabin conditioning. Fuel consumption saving has been observed as well as a sensible pollutants reduction, taking obviously into account the mechanical power required by the compressor.
Climate change is becoming more important day after day. The main actor to decarbonize the economy is the building stock, especially in the energy used for Domestic Hot Water (DHW), heating and cooling. The use of renewable energy sources to cover space conditioning and DHW demands is growing every year. While solar thermal energy can cover building heating and DHW demands, there is no technology with such potential and development for space cooling. In this paper, a new concept of combining radiative cooling and solar thermal collection, the Radiative Collector and Emitter (RCE), through the idea of an adaptive cover, which uses different material properties for each functionality, is for the first time experimentally tested and proved. The RCE relies on an adaptive cover that uses different material properties for each functionality: high spectral transmittance in the solar radiation band and very low spectral transmittance in the infrared band during solar collection mode, and high spectral transmittance in the atmospheric window wavelength during radiative cooling mode. Experiments were performed during the summer period in Lleida (Dry Mediterranean Continental climate). The concept was proved, demonstrating the potential of the RCE to heat up water during daylight hours and to cool down water during the night. Daily/nightly average efficiencies up to 49% and 32% were achieved for solar collection and radiative cooling, respectively. ; This research was funded by the Oficina de Desenvolupament i Cooperació (ODEC), the Catalan Government (AGAUR–Generalitat de Catalunya) under grant agreement 2017 SGR 659, the Spanish government (Ministerio de Ciencia, Innovación y Universidades) under grant agreement RTI2018-097669-A-I00.
Solare Kühlung ist eine Technologie, die sich noch auf dem Weg in die Wettbewerbsfähigkeit zu herkömmlichen, in der Regel mit Strom angetrieben Kompressionskältemaschinen befindet. Ein Schlüssel für die angestrebten Primärenergieeinsparungen und Kostenvorteile ist ein effizientes Betriebsverhalten der Solarkollektoren über einen Zeitraum von 20-25 Jahren. Die vorliegende Arbeit untersucht daher verschiedene thermische und hydraulische Eigenschaften von Solarkollektoren für solare Kühlung. Das übergeordnete Ziel ist es, wichtige Faktoren, die zu Leistungsminderung führen, zu identifizieren und Methoden und Techniken für eine detaillierte thermische und hydraulische Analyse der Solarkollektoren in Bezug auf diese Faktoren zu entwickeln, um schließlich praktische Richtlinien für Auslegung und Betrieb der Kollektoren vorzuschlagen. Ein beispielhaftes System ist im Bundespresseamt der Bundesregierung in Berlin installiert. Es besteht aus zwei LiBr/H2O Absorptionskältemaschinen mit je 35 KW Kühlleistung, einem Feld aus Vakuumröhrenkollektoren mit einer Gesamtfläche von 348 m2 und zwei 800-Liter-Heißwassertanks als Wärmespeicher. Zunächst wurde eine Analyse der Betriebseigenschaften des Systems auf der Grundlage von Messdaten für die thermischen Energieumsätze der verschiedenen Heiz- und Kühlanlagen und für den Stromverbrauch der elektrischen Komponenten durchgeführt. Ein spezielles Bewertungsverfahren wurde angewendet, bei welchem verschiedene zentrale Leistungsgrößen für den Betrieb des Systems wie der thermische und elektrische COP (Coefficient of Performance, entspricht Wirkungsgrad), solare Deckungsgrad und Primärenergieeinsparung im Vergleich zu einer nicht solar basierten Klimaanlage bestimmt werden. Die Ergebnisse wurden für verschiedenen Monate des Jahres 2007 bis 2009 ausgewertet und die Ergebnisse werden in Form von Kennzahlen dargestellt. Die Ergebnisse zeigen einen sehr niedrigen thermischen Wirkungsgrad der Solarkollektoren und sehr geringe Primärenergieeinsparungen. Mögliche Ursachen hierfür und Optionen zur Verbesserung werden ebenfalls diskutiert. Weiter wurde eine einfache experimentelle Vorgehensweise entwickelt, um im Labor den Einfluss des Druckes der Restgase im Inneren der evakuierten Glasröhre auf den Wärmeverlustkoeffizienten (U-Wert) einer einzelnen Vakuumröhre des Kollektors zu finden. Eine Reihe von Kollektorröhren gleicher Geometrie wurden nach dem Zufallsprinzip aus der genannten Solaranlage ausgewählt und einzeln getestet. Die Messergebnisse zeigen, dass die meisten der untersuchten Kollektorröhren höhere Wärmeverlustkoeffizienten haben als erwartet und sich entsprechend eine erhebliche Menge an Gas im Inneren der Glashülle befinden muss. Für gleiche Bedingungen wurde ein theoretisches Modell für die näherungsweise Berechnung der U-Werte entwickelt. Das theoretische Modell wurde mit den experimentellen Ergebnisse für eine bei Atmosphärendruck mit Luft gefüllte Röhre verglichen und eine gute Übereinstimmung wurde festgestellt. Dann wurde der Einfluss des Gasdruckes für verschiedene Gase untersucht. Mögliches Vorhandensein von Luft, Wasserstoff, Helium und Argon wird diskutiert. Eine CFD-Analyse (Computational Fluid Dynamics) wurde durchgeführt, um die Druckverluste von den Verteiler- und Sammlerelementen (T-Stücke) zu finden, die die parallel geschalteten Vakuumröhren im Solarkollektor wärmeträgerseitig verbinden. Simulationen wurden für eine Reihe von Strömungsverhältnissen und Reynolds-Zahlen durchgeführt und für die sich ergebenden Druckverlustkoeffizienten wurden Näherungsgleichungen entwickelt. Mit Hilfe der Ergebnisse für den laminaren Bereich wurde die isotherme und nicht-isotherme Strömungsverteilung im Kollektor, bestehend aus 60 parallelen Vakuumröhren, die entweder in U-Konfiguration oder in Z-Konfiguration verbunden sind, abgeschätzt. Die Ergebnisse der U-Konfiguration konnten mit experimentellen Daten verglichen werden und es ergibt sich eine gute Übereinstimmung. Ein stationäres Modell wurde entwickelt, um die thermische Leistungsfähigkeit einer einzelnen Vakuumröhre eines Solarkollektors mit koaxialer Wärmeträgerführung (Direct Flow-Typ) sowohl bei einphasiger als auch bei zweiphasiger Strömung (partial stagnation) zu studieren. Ein System von Gleichungen, welches die verschiedenen Wärmeübertragungsmechanismen und Strömungsverhältnisse beschreibt wurde aufgestellt, diskretisiert und iterativ gelöst. Für den Fall eines intakten Vakuums (10-5 mb) weicht die berechnete Wirkungsgradkurve für einphasige Strömung bei steigender Kollektortemperatur wesentlich von Experimenten ab, während sie für den Fall von Wärmeleitung durch Restgas innerhalb des Glaskolbens bei geringem Druck (< <1mb) durch die entsprechende Erhöhung der Wärmeverlustkoeffizienten mit den Experimenten übereinstimmt. Die Ergebnisse der Modellierung zeigen, dass der Kollektorwirkungsgrad mit abnehmendem Durchfluss abnimmt. Bei den dann erreichten hohen Temperaturen kann lokal Zweiphasenströmung durch Sieden und Rückkondensation auftreten. Sobald die Flüssigkeit den Siedepunkt bei einem bestimmten Massenstrom erreicht ist keine signifikante Reduktion des Wirkungsgrades mehr zu beobachten, was in Übereinstimmung mit Experimenten ist. Schließlich werden für optimale Leistungsfähigkeit eines ganzen Kollektorfeldes aus parallel geschalteten Vakuumröhrenkollektoren eine Reihe von Leitlinien und Regeln aufgestellt, die während Auslegung und Betrieb des Kollektorfeldes zu beachten sind. ; Solar cooling is an emerging technology and in a process of development to be competitive with the conventional systems generally based on electricity driven vapor compression cooling machines. Efficient performance of the solar collectors consistently over a time span of 20-25 years is the key towards achieving the required primary energy savings and cost benefits. The present research work investigates various thermal and hydraulic aspects of the solar collectors used for a solar cooling application. The overall aim is to identify major factors contributing to solar collector's performance reduction and to develop methods and techniques for a detailed thermal and hydraulic analysis of solar collectors pertaining to these factors, in order to suggest the optimal operating guidelines to be implemented in practice for solar collector design and operation. An exemplary system is installed at the Federal Press Information Center, Berlin. It consists of two LiBr/H2O absorption chillers with 35 KW cooling capacity for each one, a collector field made up of vacuum tube collectors having a total gross area of 348 m2, and two 800 liters of heat storage tank. Initially, the overall performance monitoring of the system was carried out, based on the on-field measured data for thermal energies of the various heating/cooling components of the system and electricity consumption of the electrical components. A monitoring procedure was then implemented and various key performance factors related to the system operation are determined such as total electrical COP, solar heat management efficiency, fraction of primary energy savings compared to a non-solar based air-conditioning system. The results are evaluated for various months of year 2007 to 2009, and the outcomes are presented in the form of performance figures. The results showed quite a low thermal efficiency of the solar collector and primary energy savings. Possible causes of the reduced collector performance and options for improvement are discussed also. A novel simplified experimental strategy is devised to find the overall heat loss coefficient (U-value) of an individual vacuum collector tube in the laboratory, with regard to the pressure of the remaining gas inside the evacuated glass envelope. A number of collector tubes of same geometry are randomly selected from an installation of a solar based air-conditioning system and tested individually in the laboratory for the determination of the U-value. Measurement results indicate that most of the examined collector tubes have higher overall heat loss coefficients than expected corresponding to a significant amount of gas inside the glass envelope. For the same conditions, an approximate theoretical model is developed for the evaluation of the U-value. The theoretical model is validated against the experimental results for a collector tube having air inside the glass cover at atmospheric pressure and found to be in close agreement. Then, the influence of gas pressure is studied for various gases. Possible presence of air, hydrogen, helium and argon is discussed. A computational fluid dynamics (CFD) analysis has been conducted to find the pressure losses for dividing and combining fluid flow through a tee junction of a solar collector manifold. Simulations are performed for a range of flow ratios and Reynolds numbers and equations are developed for pressure loss coefficients at junctions. A theoretical model based on successive approximations then is employed to estimate the isothermal and non-isothermal flow distribution in laminar range through a collector consisting of 60 vacuum tubes connected in parallel in a reverse (U-configuration) and parallel (Z-configuration) flow arrangement. The results are in reasonable agreement with the available experimental results for U-configuration. An analytical steady state model is developed to study the thermal performance of an individual vacuum tube solar collector with coaxial piping (direct flow type) incorporating both single and two-phase flows. A system of equations which describe the different heat transfer mechanisms and flow conditions was established, discretized, and solved in an iterative manner. For the case of good vacuum condition (10-5 mb) the calculated efficiency curve for single phase flow deviates significantly from the experiments with increasing collector temperature, but agrees well for the case of gas conduction inside the glass envelope at very low pressure (« 1mb) due to the corresponding increase in overall heat loss coefficient (U-value). For two-phase flow, the occurrence and propagation of flow boiling and condensation inside the collector piping under saturated condition is hypothesized. The modeling results indicate that for all-liquid-single-phase fluid flow, the collector efficiency decreases with decreasing mass flow rate. Once the fluid reaches the boiling point at a certain mass flow rate, no significant reduction in efficiency is observed anymore, which is in accordance with the experimental study. Finally, performance predictions are made for the whole collector consisting of a number of vacuum tubes connected in an array. This leads to devise a set of guidelines and strategies, which needs to be taken care off during design and/or operation phase to optimize the collector operation.
