Silicon oxynitride multilayers as spectrally selective material for passive radiative cooling applications

Silicon oxynitride has optical properties that makes it attractive as radiative cooling material. Because of the interface contributions, multilayered configurations provide a very promising way to enhance the device performances. Three silicon oxynitride multilayers deposited onto aluminum-coated glass substrate were prepared and tested. A broadening of the absorption peak within the atmospheric window was observed. Radiative cooling parameters _S2^H and ν^H were deduced from infrared reflection measurements. They correspond to temperature drops of 52, 48 and 56°C (from ambient temperature of 27°C), and cooling powers of 125, 118 and 119 Wm⁻². Temperature drops and cooling power remain competitive for practical uses, even after non radiative exchange processes are taken into account.     

Materials for radiative cooling to low temperature

The clear sky can act as a heat sink. Cooling to low temperatures is possible with materials which are strongly emitting in the 8–13 μm band and non-absorbing elsewhere. In this paper we discuss the resource for radiative cooling and its implementation with thin solid films (SiO_0.6N_0.2 coatings on Al) and with slabs of certain gases (C₂H₄, C₂H₄O and NH₃ backed by Al). Results are given on spectrophotometric infrared reflectance and transmittance, computed parameters which govern the predicted cooling performance, and some preliminary field tests.     

Thin semiconductor films for radiative cooling applications

Passive cooling systems use simple low-cost techniques to provide summer comfort in warm climates and can also be used to keep food, liquids and other materials at temperatures below ambient. Radiative cooling devices require a convective shield that should reject solar radiation but be transparent to mid-IR thermal radiation. In this paper, chemical solution deposition of thin semiconductor films (PbS and PbSe) onto polyethylene foils for radiative cooling applications is described. Optical and structural characterizations of the films were performed using UV–Vis, FTIR spectroscopies, X-ray diffraction and electron microscopy. Additionally, pigmented shields, which exhibit good radiation scattering properties, were prepared by incorporation of ZnS or ZnO into polyethylene. We also studied a combination of pigmented polyethylene foils coated with thin films of PbS that show improved optical properties for cooling purpose.     

Separation of variables solution for non-linear radiative cooling

A separation of variables solution has been obtained for transient radiative cooling of an absorbing-scattering plane layer. The solution applies after an initial transient period required for adjustment of the temperature and scattering source function distributions. The layer emittance, equal to the instantaneous heat loss divided by the fourth power of the instantaneous mean temperature, becomes constant. This emittance is a function of only the optical thickness of the layer and the scattering albedo; its behavior as a function of these quantities is considerably different than for a layer at constant temperature.     

Photovoltaic-thermal collectors for night radiative cooling of buildings

A new photovoltaic-thermal (PVT) system has been developed to produce electricity and cooling energy. Experimental studies of uncovered PVT collectors were carried out in Stuttgart to validate a simulation model, which calculates the night radiative heat exchange with the sky. Larger PVT frameless modules with 2.8 m² surface area were then implemented in a residential zero energy building and tested under climatic conditions of Madrid. Measured cooling power levels were between 60 and 65 W m⁻², when the PVT collector was used to cool a warm storage tank and 40-45 W m⁻², when the energy was directly used to cool a ceiling. The ratio of cooling energy to electrical energy required for pumping water through the PVT collector at night was excellent with values between 17 and 30. The simulated summer cooling energy production per square meter of PVT collector in the Madrid/Spain climatic conditions is 51 kWh m⁻² a⁻¹. In addition to the thermal cooling gain, 205 kWh m⁻² a⁻¹ of AC electricity is produced under Spanish conditions. A comparative analysis for the hot humid climate of Shanghai gave comparable results with 55 kWh m⁻² a⁻¹ total cooling energy production, mainly usable for heat rejection of a compression chiller and a lower electricity production of 142 kWh m⁻² a⁻¹.     

The radiative cooling of selective surfaces

We have realized a selective surface with optical properties matched to the atmospheric window 8–13 μm. With respect to a black radiator, this surface is considerably more effective for cooling by exposition to the clear sky: in particular, a cooling effect is obtained also during the day. Theoretical results and experimental data are presented.     

Condensation of water by radiative cooling

Atmospheric humidity can be condensed as dew and used for example in small-scale irrigation. In arid locations, the most favourable conditions for dew collection persist in the late night and around sunrise. We study the possibility to use a dew collector for condensing atmospheric water vapour by exploiting the effect of radiative cooling. In particular, we study pigmented polymer foils with high solar reflectance and high thermal emittance. Suitable pigments are a mixture of TiO₂ and BaSO₄ particles or a novel SiO₂/TiO₂ composite. We calculate the condensation rate under different climatic conditions and report on initial field tests.     

A robust convection cover material for selective radiative cooling applications

The enhanced cooling of exposed surfaces by radiative heat loss to the cold sky was investigated using a variety of commonly available materials. Zinc sulphide was identified as a durable substance suitable for the construction of convection covers for radiative cooling radiators. With respect to polyethylene, the most commonly used convection cover to date, the new material is mechanically stronger, impervious to damage by solar ultraviolet and in practical thicknesses is more transparent in the 8-14 μm waveband. Use of this window material with a previously proposed selective radiator material, a form of anodised aluminium that reflects radiation at wavelengths shorter than 8 μm allows for the economical production of an effective selective radiator system. Measurements were made on simple radiator plates and convection covers.     

Modeling of water spray evaporation: Application to passive cooling of buildings

The use of water droplet evaporation in shower towers and passive downdraft evaporative cooling needs the estimation of the time needed to completely evaporate the drops. To solve this problem, a cellular approach is proposed in which the spray is considered as a pile of rigid spheres of equal size; each sphere has multiple layers and contains a drop in its center. The evaporation takes place gradually from the superficial layer towards the internal layers. Parametric studies show the influence of each variable on the evaporation time of the droplets. The model may be used for sizing passive evaporative cooling systems and towers for buildings using the passive evaporative down draught effect. A building equipped with a shower tower has been tested in the framework of the European project PDEC/JOULE in Catania (Italy). The spraying system was successfully sized by using the model proposed in this paper.     

A study of a polymer-based radiative cooling system

A radiative cooling system consisting of unglazed flat plate radiators, water as heat carrier and a reservoir is presented. The radiators are twin-wall sheets made of a modified PPO (polyphenylenoxid) resin, which are proposed as low cost roof integrated modules. The thermal performance of a system with a radiator aperture area of 5.3 m² and reservoir volume of 280 l has been investigated in experiments for Oslo climate. A parameterisation for the cooling performance of a tilted radiator surface for clear and cloudy atmospheres is proposed and applied to model the experimental results. The impact of the tilt angle, the aperture area and the reservoir volume on the cooling performance has been studied in simulations. The feasibility of a radiative cooling system designed for a single-family house at southern latitudes has been modelled. Except for mid-summer ambient temperature and high relative humidity, the simulations show that the radiative cooling system seems to cover the demand.     

Application of passive downdraught evaporative cooling (PDEC) to non-domestic buildings

The applicability of Passive Downdraught Evaporative Cooling (PDEC) for reducing energy consumption in hot dry climates is reviewed. A new EC Joule project explaining the application of PDEC in non-domestic buildings is described. The building performance assessment methodology which employs dynamic thermal simulation programs for thermal analysis, and computational fluid dynamics (CFD) codes for airflow modelling is discussed. The role which wind tunnel tests and field measurements have in producing improved models is noted. Preliminary results from CFD benchmark trials are presented.     

Simplified model of transient radiative cooling of spherical body

The cooling of a spherical body in case when radiation is the only heat transfer mechanism between the body surface and the environment has been considered. A mathematical process model employing an own approximate kinetic equation of heat conduction has been worked out. Thus, the process has been described by an ordinary differential equation combined with an algebraic one. The proposed simplified model has been compared with the exact solution and with approximate relations presented by Su [4]. It has been found that the proposed model is accurate and employable for both long and short times as well as for any relations between outer and inner heat transfer resistances.     

Complete condensation in a vertical tube passive condenser

An experimental study is performed for the steam condensation in a vertical tube where steam is completely condensed. A condenser tube is submerged in a water pool where the heat from the condenser tube is removed through boiling heat transfer. The experiment data showed that the operating pressure is uniquely determined by inlet steam flow rate for the complete condensation. The condensation heat transfer rate increases and the condensation heat transfer coefficient decreases with the system pressure. For the condenser submerged in a saturated water pool, strong primary pressure dependency was observed on the condensation heat transfer.     

Review of passive systems and passive strategies for natural heating and cooling of buildings in Libya

By proper passive design concepts which essentially consist of collection, storage, distribution, and control of thermal energy flow, an energy saving of 2.35% of the world energy output is possible. The basic methods of heating and cooling of buildings are solar radiation, outgoing longwave radiation, water evaporation, and nocturnal radiation cooling. A Trombe-Michel wall consists of a large concrete mass, exposed to sunlight through large, south-facing windows; it is used for heating buildings. Solar absorption cooling and solar dehumidification and evaporative cooling are two approaches that utilize solar energy for the generation of the working fluid and the cooling of dwellings. Outgoing longwave radiation is the most practical way of cooling buildings in desert climates and is effective on roof surfaces, emitting the radiations from the surface of earth to the atmosphere and to outer space. Water evaporation in desert coolers is the usual method of cooling in arid regions. Nocturnal radiation both heats in winter and cools in summer, in suitable climates, and does so with no nonrenewable energy other than a negligible amount required to move the insulation twice a day. The study of 24 different locations in Libya divides the country into regions with distinct passive strategies. The northern region and the Mediterranean coast need passive heating. The buildings in this region should restrict conductive heat flow, prevent infiltration and promote solar heat gains. The southern region, a part of the Sahara desert, needs passive cooling. The buildings in this region need high thermal mass and should promote natural ventilation, restrict solar heat gains and encourage evaporative and radiant cooling. The difficulties encountered in passive solar design are the large exposed area required with suitable orientation for the collection of energy and the large space requirement for the storage of thermal energy. This paper reviews these passive systems and discusses suitable strategies to be adopted for Libya.     

Indoor temperature reduction by passive cooling systems

This chapter will present experimental data from studies in test cells, and from monitoring indoor temperatures in un-occupied and in occupied buildings, when data was available, regardless of the dates of the research, not just recent research. In many cases where sufficiently measured data of the outdoor and the indoor temperatures was available, formulas have been generated, expressing the indoor temperatures of the building or the test model as a function of the outdoor temperatures. In cases where long-term monitoring has been conducted, predicting formulas were generated: data from one period was used for the generation of the formulas while the data other period was used for their validation. All the formulas represent the experimental data and conditions of the various studies. They are not intended to be used for general prediction of the performance of the tested passive cooling system.     

Water distiller/condenser by radiative cooling of ambient air

A water distiller/condenser system by radiative cooling of ambient air is designed. A mathematical model is proposed to describe the thermal performance of the radiative distiller. The results of the mathematical model predicts condensation rates within the range 3–7 litre/m² night. Also, the effects of different design parameters on the distiller performance are investigated.     

Development of a solar-powered passive ejector cooling system

This paper describes the development of an ejector refrigeration system that is powered by solar thermal energy. The cooling system contains no active parts and is therefore deemed passive. Water is used as the refrigerant though other natural refrigerants could be used for lower temperature operation. A prototype system was built with a nominal cooling capacity of 7 kW. This system was laboratory tested and then installed in an existing office in Loughborough, UK. The system has operated with a COP of up to 0.3 at this location. The system is also able to provide up to 20 kW of heating to the building during the winter.