Solar properties of some building elements

An accurate set of measurements is presented of solar absorptance (_s) for wavelengths (0.3–2.0 μm) and total hemispherical thermal emittance (_h) for i.r. wavelengths (3–30 μm) of some building elements. The _s values vary from 0.19 to 0.93 ± 0.02, depending upon the nature, colour and texture of materials. The _h values for most of the building elements are high and vary from 0.80 to 0.92 ± 0.02. Weathering, rusting, anodization and wetting of materials considerably increase their solar absorptance and total hemispherical thermal emittance.     

Effect of solar absorptivity and emissivity of exterior surfaces on the thermal performance of a building

The effects of the solar absorptivity and the thermal emissivity of exterior building surfaces on the indoor air temperature of a one room building are evaluated in terms of the discomfort index. The thermal performance of the building has been investigated for four different climates prevalent in India, namely, composite (New Delhi), hot and dry (Jodhpur), cold and humid (Srinagar), and cold and dry (Leh). The results confirm the common-sense view that the building surface should have low solar absorptivity and high thermal emissivity in hot climates and high solar absorptivity and low thermal emissivity in cold climates for indoor thermal comfort conditions.     

Parametric study of the overall performance of a solar hot water system

The effect of varying individual parameters on the overall performance and feasibility of a solar hot water system has been investigated by a mathematic model which employs the concept of a decay constant and feasibility factor. TRNSYS results were used as inputs to apply the model. Parameters investigated include emittance of the collector plate, collector plate absorptance, transmittance absorptance product, storage tank set temperature, mass flow rate, load, and ratio of tank volume to collector area. It was found that the model is a useful tool for solar system design. Specific examples are detailed for illustration purposes.     

Coloured paints based on iron oxide and silicon oxide coated flakes of aluminium as the pigment, for energy efficient paint: optical and thermal experiments

Energy efficient coloured paint coatings utilising flaky aluminium pigment with either single layer (Fe₂O₃) or double layer (Fe₂O₃ on SiO₂) interference coatings are optically and thermally characterised. Similar pigments with coatings on flaky dielectric particles and standard paints of similar colour are compared. Data presented includes hemispherical and specular reflectance spectra across visible and infra red wavelengths, thermal emittance from an emissometer and light spreading data from a photogoniometer. Solar absorptance, and colour show that, as theoretically predicted, and provided flaky metal pigments are used, a wide range of colours combined with a much lower solar absorptance than traditional paints of similar colour is achieved. Superior thermal performance to ordinary paints with similar colour is thus possible and is demonstrated for two such coloured layers via controlled heating-cooling studies under both an indoor lamp and outdoor clear sky solar illumination. It is seen that heat gains from the sun and associated cooling loads can be reduced by up to 50% for most colours, which is of some benefit for cars and metal roofs in hot climates.     

High performance W–AlN cermet solar coatings designed by modelling calculations and deposited by DC magnetron sputtering

High solar performance W–AlN cermet solar coatings were designed using a numerical computer model and deposited experimentally. In the numerical calculations aluminium oxynitride (AlON) was used as ceramic component. The dielectric function and then complex refractive index of W–AlON cermet materials were calculated using the Sheng's approximation. The layer thickness and W metal volume fraction were optimised to achieve maximum photo-thermal conversion efficiency for W–AlON cermet solar coatings on an Al reflector with a surface AlON ceramic anti-reflection layer. Optimisation calculations show that the W–AlON cermet solar coatings with two and three cermet layers have nearly identical solar absorptance, emittance and photo-thermal conversion efficiency that are much better than those for films with one cermet layer. The optimised calculated AlON/W–AlON/Al solar coating film with two cermet layers has a high solar absorptance of 0.953 and a low hemispherical emittance of 0.051 at 80°C for a concentration factor of 2. The AlN/W–AlN/Al solar selective coatings with two cermet layers were deposited using two metal target direct current magnetron sputtering technology. During the deposition of W–AlN cermet layer, both Al and W targets were run simultaneously in a gas mixture of argon and nitrogen. By substrate rotation a multi-sub-layer system consisting of alternating AlN ceramic and W metallic sub-layers was deposited that can be considered as a macro-homogeneous W–AlN cermet layer. A solar absorptance of 0.955 and nearly normal emittance of 0.056 at 80°C have been achieved for deposited W–AlN cermet solar coatings.     

Analyzing the effect of the longwave emissivity and solar reflectance of building envelopes on energy-saving in buildings in various climates

A dynamic computer simulation is carried out in the climates of 35 cities distributed around the world. The variation of the annual air-conditioning energy loads due to changes in the longwave emissivity and the solar reflectance of the building envelopes is studied to find the most appropriate exterior building finishes in various climates (including a tropical climate, a subtropical climate, a mountain plateau climate, a frigid-temperate climate and a temperate climate). Both the longwave emissivity and the solar reflectance are set from 0.1 to 0.9 with an interval of 0.1 in the simulation. The annual air-conditioning energy loads trends of each city are listed in a chart. The results show that both the longwave emissivity and the solar reflectance of building envelopes play significant roles in energy-saving for buildings. In tropical climates, the optical parameters of the building exterior surface affect the building energy-saving most significantly. In the mountain plateau climates and the subarctic climates, the impacts on energy-saving in buildings due to changes in the longwave emissivity and the solar reflectance are still considerable, but in the temperate continental climates and the temperate maritime climates, only limited effects are seen.     

Technoeconomic optimization of solar natural convection hybrid hot water systems

In this paper a techno-economic model for a hybrid domestic hot water system operating under natural convection mode is presented. Three modes of auxiliary energy supply viz.

• A electric heater fitted in the solar hot water tank.
• B electric heater fitted in a small water tank in series with the solar hot water tank, and
• C an instant electric heater fitted in the tap

have been considered. Taking into account the life and the capital and maintenance costs of the solar and electrical equipments, the cost of useful energy (Rs/kWh) has been calculated for different values of the collector area and the tank capacity, and thereby the optimum collector area and tank capacity (for a given demand), corresponding to minimum cost of useful energy, has been determined. From numerical calculations made for the climate of Delhi, India (a representative composite climate) corresponding to the two cases of monthly hot water demand viz. (i) constant monthly demand, and (ii) variable monthly demand, it is seen that case (C) is the most economic design of the hybrid hot water system; numerical calculations have also been made for this case corresponding to the climates of Srinagar and Madras (representing cold and hot climates). The effect of government subsidy on the optimized values of collector area, tank capacity and cost of useful energy has also been investigated for the climate of Delhi.     

The impact of optical and thermal properties on the performance of flat plate solar collectors

The impact of the optical properties on the annual performance of flat plate collectors in a Swedish climate has been estimated with the MINSUN program. The collector parameters were determined with a theoretically based calculation program verified from laboratory measurements. The importance of changes in solar absorptance and thermal emittance of the absorber, the addition of a teflon film or a teflon honeycomb, antireflection treatment of the cover glazing and combinations of these improvements were investigated. The results show that several improvements can be achieved for solar thermal absorbers. A combined increase in absorptance from 0.95 to 0.97 and a decrease in emittance from 0.10 to 0.05 increase the annual performance with 6.7% at 50 °C operating temperature. The increase in performance by installing a teflon film as second glazing was estimated to 5.6% at 50 °C. If instead a teflon honeycomb is installed, a twice as high performance increase is obtained, 12.1%. Antireflection treatment of the cover glazing increases the annual output with 6.5% at 50 °C. A combination of absorber improvements together with a teflon honeycomb and an antireflection treated glazing results in a total increase of 24.6% at 50 °C. Including external booster reflectors increases the expected annual output at 50 °C to 19.9–29.4% depending on reflector material.     

Thin film angular selective glazing

Angular selective glazing based on special obliquely deposited thin films, has considerable potential for optimizing the overall performance of a window in temperate to hot climates. In contrast to most currently used glazings for these environments it has the ability to control solar heat gain and glare while providing a clear view through the window and considerable but not excessive light gain. Various deposition techniques and film materials will be discussed and the spectral and integrated optical performance of single and double-layered systems presented for various directions of incidence. Full optical characterisation requires measurements for incidence directions over half a hemisphere. Some of this is presented but most data is confined to planes of incidence where angular selectivity is greatest as determined by deposition geometry. Two layer systems are designed to provide a low emittance and better solar control than the single layer systems while retaining good visible angular selectivity. The single layers are primarily to provide angular selective control of the visual radiation since they have moderately high transmittance in the near infrared and a high emittance. Theoretical models used to extract structural information and model the complex optical properties are introduced and their predictions assessed. Preliminary results on simulation of the performance of full size angular selective windows in comparison to other types of solar control glazing such as low e heavily tinted windows are discussed in terms of their relative impact on cooling load, lighting contribution, glare control and total energy savings.     

Selective absorbing surface for evacuated solar collector tubes

The absorptance of AlN-Al(absorbing coating)/Al solar spectrum(0.34 ≈ 2.5 μ m) selective absorbing surface, containing 8-layer AlN-Al composite materials and with distinctive or non-boundaries between the layers, deposited by sputtering technology and heat-treated at 400 °C (60min) is about 0.95 and its thermal emittance is close to 0.07 ≈ 0.08(80 °C). But on the condition that the absorptance is unchanged, the thermal emittance dropped to 0.04 for the AlN-Al coating composed of matel volum fractionic 0.42, 0.25 and 0.00 films.     

A new method to develop typical weather years in different climates for building energy use studies

Principal component analysis of 30-year long-term meteorological variables was conducted. Typical principal component years (TPCYs) were determined for Harbin, Beijing, Shanghai, Kunming and Hong Kong representing the five major architectural climates across China: severe cold, cold, hot summer and cold winter, mild, and hot summer and warm winter. In each climate zone, the TPCY was compared with the 30 individual years and the widely used typical meteorological year (TMY). The monthly principal component and the predicted total building energy consumption based on the TPCY and TMY were very close to the 30-year long-term mean estimation. TPCY for the 21st century in each of the five cities was also identified using predictions from general climate models. The TPCY approach is a good alternative to the TMY method. Firstly, predicted building energy use from TPCY is closer to the long-term estimation than that from the TMY in different climates. Secondly, because only monthly data are considered, the development of TPCY is much simpler and less time-consuming. This would have important applications in the regular updating of typical weather years for building energy studies and in the assessment of the impact of climate change on energy use in the built environment.     

Life cycle energy analysis of a residential building with different envelopes and climates in Indian context

In this paper life cycle energy (LCE) demand of a residential building of usable floor area about 85.5 m² located at Hyderabad (Andhra Pradesh), India is evaluated under different envelopes and climates in Indian context. The house is studied with conventional (fired clay) and alternative wall materials (hollow concrete, soil cement, fly ash and aerated concrete) under varying thickness of wall, and insulation (expanded polystyrene) on wall and roof. The house is modelled for five different climatic zones of India, i.e. hot and dry, warm and humid, composite, cold and moderate. Study suggests that alternative wall materials alone (without insulation) reduce LCE demand of the building by 1.5-5%. Aerated concrete (AC), as wall material, has better energy performance over other materials. LCE savings are significant when insulation is added to external wall and roof. It varies from 10% to 30% depending on the climatic conditions. Maximum LCE savings with insulation are observed for warm and humid climate and least for moderate climate. For same thickness of insulation, LCE savings are much more with roof insulation than wall insulation. But wall insulation is found to be preferable to a thicker wall. It is also observed that there is a limit for thickness of insulation that can be applied on external walls and roof from life cycle point of view. This limit is found to be about 10 cm for composite, hot and dry, warm and humid, and cold climates and 5 cm for moderate climate.     

Measurement of spectra of normal incidence absorptance of surfaces in life space

The thermal and radiative environment of human life space is characterized strongly by solar radiation and by the radiation from surrounding walls as well as by air temperature and humidity. To consider the environmental radiation quantitatively, absorptance spectra and total emittance of the surfaces should be measured properly. In this work we measure spectra of normal incidence absorptance of such surfaces in a near‐ultraviolet through infrared region of wavelength of 0.30–11 μm. On the basis of measured spectra we evaluate solar absorptance of the surfaces to respond to the needs of radiation characteristics of the surfaces in the fields of architecture, life science, and solar engineering. Objective surfaces of the measurement are surfaces of indoor and outdoor environments: cloths, indoor wall materials, painted surfaces, ceramics, bricks, outdoor wall materials, road materials, leaves, etc. We also observe the transition of the absorptance spectra of a water–wet cement surface in a drying process. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20356     

Optimization of solar absorbing three-layer coatings

The optimization of spectrally selective solar-absorbing three-layer coatings was studied from both theoretical and experimental points of view. The base layer consists of a cermet (ceramic–metal composite) material, where small metal clusters are embedded in the dielectric surrounding. The results show considerable enhancement of the solar absorption due to the metal nanoparticles. Theoretical treatment of the optical properties of the cermet was carried out using effective medium theory. The experimentally determined optical constants of calibration samples were used to calculate the film reflectance of single or multiple layers. Theoretically optimized three-layer coatings show solar absorptance value as high as 0.97 and moderate reflectance at 2.5 μm giving low thermal emittance. Experimental optimization was carried out through control of the coatings layer by layer. The metal content and the film thickness are two important parameters for cermet films. Only thickness is sensitive to the middle and top layer. The optical properties are matched in such a way that the base layer functions as main absorber, the top layer reduces the reflectance from the front surface, and the middle layer links the base and the top layers. An experimentally optimized solar absorptance of 0.97 with a thermal emittance of 0.05 was obtained, which shows that the optimization procedure is reliable.     

Computer simulation of the optical properties of high-temperature cermet solar selective coatings

A computer simulation is developed to calculate the solar absorptance and thermal emittance of various configurations of cermet solar selective coatings. Special attention has been paid to those material combinations, which are commonly used in high-temperature solar thermal applications. Moreover, other material combinations such as two-, three- and four-cermet-layer structures as solar selective coatings have been theoretically analyzed by computer simulation using three distinct physical models of Ping Sheng, Maxwell–Garnett and Bruggeman. The novel case of two-cermet-layer structure with different cermet components has also been investigated. The results were optimized by allowing the program to manipulate the metal volume fraction and thickness of each layer and the results compared to choose the best possible configuration. The calculated results are within the range of 0.91–0.97 for solar absorptance and 0.02–0.07 for thermal emittance at room temperature.     

Characterization of Ti1−xAlxN coatings with selective IR reflectivity

Ti_1−xAl_xN thin films were deposited by reactive magnetron sputtering. The obtained different stoichiometries give rise to different optical properties as the films change from metallic to dielectric. In this work the IR reflectivity of these coatings is investigated taking into account different application fields for IR selective Ti_1−xAl_xN thin films.Low Al content coatings present high reflectivity, high absorptance and low thermal emittance. High Al compositions give raise to coatings with high absorptance and high thermal emittance.The composition of the coatings was evaluated combining electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy. Scanning electron microscopy (SEM) revealed a columnar structure. Reflectance spectra for the visible and infrared spectral ranges were used to obtain the solar absorptance and thermal emittance values, used to calculate the equilibrium temperature of the coatings.The thermal stability in air from 300 to 600 °C was also evaluated.     

Emittance from stagnation temperature study

Variation of stagnation temperature of various coatings kept in an insulated one glass cover box is theoreticall analysed. This shows that it is possible to make an approximate estimate of thermal emittance of the coating of known absorptance from the knowledge of stagnation temperature, ambient temperature and solar insolation. Such estimate of emittance for several black painted, black chrome and structured nickel coated samples are compared with their emittance values obtained from reflectance data.     

High performance selective surfaces from stainless steel for the photothermal conversion of solar energy

Chemical and electrochemical working processes for the preparation of selective absorbers on stainless steels are described. These methods have allowed conversion coatings to be obtained which exhibit high solar absorptance (_s 0.95) and low thermal emittance (₂₀ 0.20) and excellent durability. The relationships between the chemical treatment conditions and the optical properties of the coatings are discussed. The surface analysis (SIMS, ESCA) shows that the coating is heterogeneous and presents, with depth, varying proportions of metallic elements in various oxidation states (oxides, sulphides and metals).     

Radiation characteristics of honeycomb solar collectors

A simple closed-form expression for the infrared emittance and the solar absorptance of honeycomb solar collectors has been obtained in terms of the passage transmittance function. The predicted results agree well with the existing data of infrared emittance for thin-walled square-cell honeycomb collectors in vacuum. A new concept of double-honeycomb structure is also introduced and analyzed. This concept provides considerable flexibility in various designs of honeycomb collectors.     

Solar assisted biogas plants III: Energy balance of fixed dome biogas plants and enhancement of production in winters

This paper presents a thermal model of a fixed dome biogas plant, with or without hot charging and having the ground surface above the plant, blackened and glazed. The rate of energy transfer, between the ground and the plant has been determined by electrical simulation experiments, using a small scale model of the plant (made of copper), suspended in copper sulphate solution. For hot charging a shallow solar pond, built on the ground has been considered. The monthly variation of the slurry temperature, corresponding to 1, 4, 8, 15 and 30 m³ plants have been evaluated for Madras, New Delhi and Srinagar, which correspond to typical hot, composite and cold climates; the following cases have been considered:

• 1 Bare plant, without hot charging and blackening/glazing of the ground, above the plant.
• 2 Plant, charged with hot slurry from a shallow solar pond.
• 3 Plant, with ground above blackened and glazed.
• 4 Plant, with ground above blackened and double glazed and also provision for heating the slurry by burning a part of the produced biogas.

It is seen that blackening and single glazing of the ground above the plant is able to maintain the slurry temperature between 28 and 35°C throughout the year in composite climate such as Delhi; in cold climates even double glazing alone is not enough for that purpose. Hot charging from a shallow solar pond alone is not sufficient to maintain satisfactory slurry temperatures in winters of Delhi and of course Srinagar. In hot climates the slurry temperature is sufficiently high, except for a couple of months, when either hot charging from a shallow solar pond or blackening/glazing of the ground above is sufficient to have the slurry temperature in the desirable range. In cold climates such as Srinagar, the desired slurry temperatures can be maintained if a part of the gas produced is burnt for heating the slurry, in addition to blackening and double glazing of the ground above the dome. Theoretical predictions of slurry temperatures have been compared with the field data recorded for an 8 m³ digester at the Energy Complex, Masoodpur, Delhi; the agreement is satisfactory.