Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions

The impact from using cool roof coatings on the cooling and heating loads and the indoor thermal comfort conditions of residential buildings for various climatic conditions is estimated. The energy cooling loads and peak cooling demands are estimated for different values of roof solar reflectance and roof U-value. The results show that increasing the roof solar reflectance reduces cooling loads by 18–93% and peak cooling demand in air-conditioned buildings by 11–27%. The indoor thermal comfort conditions were improved by decreasing the hours of discomfort by 9–100% and the maximum temperatures in non air-conditioned residential buildings by 1.2–3.3 °C. These reductions were found to be more important for poorly or non-insulated buildings. For the locations studied, the heating penalty (0.2–17 kWh/m² year) was less important than the cooling load reduction (9–48 kWh/m² year). The application of cool roof coatings is an effective, minimal cost and easy to use technique that contributes to the energy efficiency and the thermal comfort of buildings.     

The effects of emissivity on the performance of steel in furnace tests

The role that surface emissivity plays in the standard furnace test (BS476) is considered for steel sections. Steel samples coated with either a low-emissivity paint or a high-emissivity paint were subjected to furnace tests and cone calorimeter tests in order to quantify the degree to which emissivity affects performance. The cone calorimeter experiments were designed primarily to determine the emissivity of the coatings and to compare with the manufacturer's estimates. However, a welcome additional benefit of this analysis was an estimate of the average convection heat transfer coefficient h for horizontal test specimens in the cone calorimeter. Our measurements suggest that h has been significantly underestimated (in some cases by at least 50%) in the literature to date. Most studies appear to assume a value for h that is close to the value for free convection for a hot plate with hot surface uppermost (something in the region 10–15 W m⁻² K⁻¹). Our results suggest that a figure closer to 28 W m⁻² K⁻¹ is more appropriate. The furnace tests showed that emissivity has a low-order effect on performance and so we are able to conclude that convective heat transfer is dominant in these situations.     

Performance analysis of a simple roof cooling system with irrigated soil and two shading alternatives

The use of passive cooling on roofs holds a significant unfulfilled potential in hot-arid regions. In this study, the contribution of a watered soil with two types of shading for roof cooling was assessed. Two test cells of approximately 4 m² in area and 2.5 m high were monitored during the summer season. Both cells were covered with a 16-cm layer of soil. One was untreated while the other was watered and shaded consecutively by means of an overhead shading mesh and a layer of lightweight gravel. Temperature profiles were measured across the section of each roof, from the top surface of the soil to the ceiling inside the chamber, and embedded heat flux plates were also used to evaluate the cooling effect. A comparison of the two shading strategies demonstrated that while the mesh provided more cooling over a daily cycle, the daytime cooling potential, which is crucial in a desert climate, was higher with lightweight gravel.     

Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate

Green roofs, or vegetated roofs, can reduce heat flux magnitude through a building envelope as a result of insulation provided by the growing medium, shading from the plant canopy, and transpirational cooling provided by the plants. This study quantifies the thermal properties of an inverted 325 m² retro-fitted extensive green roof versus a traditional gravel ballasted inverted roof in a Midwestern U.S. climate characterized by hot, humid summers and cold, snowy winters. In autumn, green roof temperatures were consistently 5 °C lower than corresponding gravel roof temperatures. Even during chilly and moist conditions, the heat flux leaving the building was lower for the green roof than the gravel roof. Temperatures at the top of the insulation layer were more variable for both green roof and gravel roof on winter days with no snow cover than on days with snow cover. Variation in temperatures between roof types in spring was similar to those in autumn. Peak temperature differences between gravel and green roof were larger in summer than other seasons (sometimes by as much as 20 °C). Over the course of a year (September 2005-August 2006), maximum and minimum average monthly temperatures and heat fluxes were consistently more extreme for the gravel roof than the green roof.     

Determining of heat balance design criteria for laying hen houses under continental climate conditions

This study focuses on the heat balance status of laying hen houses in regions with continental climate. The material consists of 45 laying hen houses from 27 commercial farms selected from the survey area where continental climate prevails. These laying hen houses differ from each other with respect to capacity, planning system and materials used in construction. First observations were conducted on the size and dimensions of laying hen houses as well as construction materials used, insulation, heat loss factors, ventilation capacity, ground space per hen and total size of laying hen house in order to assess the sufficiency of heat balance. Then, seven laying hen house models were developed. These models were developed by considering the present situation in operating laying hen houses, relevant literature, features of continental climate and suggestions made by firms manufacturing laying hen house construction materials in Turkey. These models give heat conduction coefficients that will prevent moisture concentration and ensure heat balance under continental climate conditions and suggest different sets of materials that can be used on walls and roofs. At the end of the study, under the condition of no moisture on surface of structural components and in areas where the indoor and outdoor temperatures are 25.3 °C and 20.2 °C, respectively, maximum total heat conduction coefficients are calculated to be between 1.38 and 1.73 Kcal/m² °C h. According to the features of area and housing, for providing heat balance, total heat conduction coefficients requirements are calculated to be between 0.62 and 2.08 Kcal/m² °C h for walls, 0.33 and 1.62 Kcal/m² °C h for roofs. In research area, minimum ventilation capacities are determined as 0.72 m³/h hen for carbon dioxide balance and, according to outdoor temperature, as 0.83–1.20 m³/h hen for water vapor balance. Heat loss factors are calculated to be between 0.10 and 0.15 Kcal/°C h hen. We believe that these suggestions will greatly facilitate the work of project engineers in the design of laying hen houses in regions and areas with continental climate.     

An investigation of sensible heat fluxes at a green roof in a laboratory setup

During the last few years, several models have been proposed for the calculation of green roof thermal behavior, but the validation studies of such models are lacking a comprehensive set of highly accurate data. In this study, an experimental laboratory setup was used to create different environmental conditions and to measure sensible heat fluxes to/from a vegetated roof assembly. This experimental setup has been successfully used for different wind velocities (0–3 m/s) to create free and forced convection conditions around green roof tested samples. Furthermore, our study proposed a “basic model” for calculations of the convective heat transfer at green roof assemblies, which is a modified version of the Newton’s cooling law, calibrated and then validated with different sets of data. For forced convection flow regimes, the proposed “basic model” resulted in RMSE (Root Mean Square Error) of 11 W/m² and R² value of 0.81. Similarly, the model provided RMSE of 6.6 W/m² and R² of 0.90 for sensible heat fluxes with free convection conditions. In the future, this model will be used in on-site experimental studies to understand its performance under wind conditions that exhibit a much wider range than the studied velocity range near the leaf canopy.     

Thermal characteristics of a double-glazed external wall system with roll screen in cooling season

A double-skin system (double-glazed external wall) is an effective passive system that can be used to decrease solar heat gain into buildings. Detailed information on the thermal distribution of double-skin facades is necessary to design better systems that can provide thermal comfort and conserve energy. In this study, the three-dimensional thermal characteristics of double-skin facades that had the ventilation opening installed partially and were screened partially by the adjacent buildings were investigated by field measurements. To that end, field measurements were carried out on the double-skin exterior wall (9.4 m high and 27.0 m wide) installed in an atrium located in the west of an existing building during cooling period for typical summer conditions. Maximum air change rate of natural ventilation through the bottom opening up to the top opening is about 20–25 [1/h], the reduction ratio of total solar heat gain compared with those of non-natural ventilation is about 25%. The exhaust solar heat gain is about 100 W/m² per inner glass surface area of the double-skin facades. Air temperature distribution of air space in the double skin was ranged from 30 °C to 44 °C, and heat gain difference ranged from 50 W/m² to 130 W/m². The influence of the ventilation openings and the shade conditions on temperature distribution of double skin is found to be significant and the double-skin system was verified to reduce the cooling loads effectively.     

Size-fractionated (1 → 3)-β-D-glucan concentrations aerosolized from different moldy building materials

Release of submicrometer-sized fungal fragments (< 1.0 μm) was discovered in earlier studies, which investigated the aerosolization of spores from moldy surfaces. However, the contribution of fungal fragments to total mold exposure is poorly characterized. The purpose of this study was to investigate the size-fractionated concentrations of particulate (1 → 3)-β-D-glucan and numbers of particles aerosolized from the surface of artificially mold-contaminated materials using a novel sampling methodology. Aspergillus versicolor and Stachybotrys chartarum were grown on malt extract agar and building materials (ceiling tiles and gypsum board) for one to six months. Fungal particles released from these materials were collected size-selectively by a newly developed Fragment Sampling System, and (1 → 3)-β-D-glucan in air samples was analyzed by Limulus Amebocyte lysate (LAL) assay. The concentrations of (1 → 3)-β-D-glucan varied from 0.4 × 10⁰ to 9.8 × 10² ng m^− 3 in the fragment size and from 1.0 × 10¹ to 4.7 × 10⁴ ng m^− 3 in the spore size range. Numbers of submicrometer-sized particles aerosolized from 6-month old cultures were always significantly higher that those from 1-month old (P < 0.001). This can be attributed to increased dryness on the surface of material samples and an increase in fungal biomass over time. The average fragment to spore ratios both in particle numbers and (1 → 3)-β-D-glucan mass were higher for S. chartarum than for A. versicolor. The results indicate that long-term mold damage in buildings may lead to increased contribution of fragments to the total mold exposure. Therefore, the health impact of these particles may be even greater than that of spores, considering the strong association between numbers of fine particles and adverse health effects reported in other studies. Furthermore, the contribution of fragments may vary between species and appears to be higher for S. chartarum than for A. versicolor.     

Performance assessment of low-energy buildings in central Argentina

This paper summarizes the results obtained from the energy and thermal performance assessment of residential and non-residential low-energy buildings that were designed to minimize fossil energy use. They are located in the province of La Pampa, central Argentina, in a temperate continental climate that shows extreme hot and cold records during the summer and winter seasons, respectively. The common applied technologies for saving energy were passive solar heating, natural ventilation for cooling and daylighting. The glazing area in the principal functional spaces facing to the North oscillates between 11 and 17% of the building useful areas. All the studied buildings are massive, with the exception of an auditorium that was designed with a lightweight insulated technology. The mean thermal transmittance of the envelope is 0.45 W/(m² K). Double glazing and hermetic carpentry were used to reduce thermal losses (U-value = 2.8 W/(m² K)). The volumetric heat loss coefficient (G-value) oscillates between 0.90 and 1.00 W/(m³ K). During the design and thermal simulation convective-radiative heat transfer coefficients were estimated through a dimensional equation (h = 5.7 + 3.8 ws, wind speed). On internal surfaces, convective-radiative heat transfer coefficients of 8 and 6 W/(m² °C) (for surfaces with and without solar gain, respectively) were applied. The monitoring process provided information on the energy and thermal behaviour under use and non-use conditions. The measured value of energy consumption was similar to the expected value that was used during the pre-design stage. Building technologies work well during the winter season, allowing 50–80% of energy savings. However, overheating is still an unresolved problem during the summer. Interviews with occupants revealed that they need both, information about functional details, and good-practice guidance to manage thermal issues of the building. In most cases, the annual consumption of energy was lower than those established by the Low Energy Housing German Standards and the Minirgie Switzerland Certificate. Despite their relative cost increase during the last years, the use of insulation technology and the application of passive solar devices involved an extra cost of only 3% in our works. Provided the expected depletion of natural gas production in the coming decade, the importance of applying energy-efficiency guidelines will increase very soon in Argentina in order to match the requirements of a new national energy matrix.     

Energy partition and conversion of solar and thermal radiation into sensible and latent heat in a greenhouse under arid conditions

For a greenhouse thermal analysis, it is essential to know the energy partition and the amount of solar and thermal radiation converted into sensible and latent heat in the greenhouse. Factors that are frequently needed are: efficiency of utilization of incident solar radiation (π), and sensible and latent heat factors (η and δ). Previous studies considered these factors as constant parameters. However, they depend on the environmental conditions inside and outside the greenhouse, plants and soil characteristics, and structure, orientation and location of the greenhouse. Moreover, these factors have not yet been evaluated under the arid climatic conditions of the Arabian Peninsula.In this study, simple energy balance equations were applied to investigate π, η and δ; energy partitioning among the greenhouse components; and conversion of solar and thermal radiation into sensible and latent heat. For this study, we used an evaporatively cooled, planted greenhouse with a floor area of 48 m². The parameters required for the analysis were measured on a sunny, hot summer day. The results showed that value of π was almost constant (≅0.75); whereas the values of η and δ strongly depended on the net radiation over the canopy (R_na); and could be represented by exponential decay functions of R_na.At a plant density corresponding to a leaf area index (LAI) of 3 and an integrated incident solar energy of 27.7 MJ m⁻² d⁻¹, the solar and thermal radiation utilized by the greenhouse components were 20.7 MJ m⁻² d⁻¹ and 3.74 MJ m⁻² d⁻¹, respectively. About 71% of the utilized radiation was converted to sensible heat and 29% was converted to latent heat absorbed by the inside air. Contributions of the floor, cover and plant surfaces on the sensible heat of the inside air were 38.6%, 48.2% and 13.2%, respectively.     

Heterogeneous photocatalytic removal of toluene from air on building materials enriched with TiO2

This paper reports the potential of heterogeneous photocatalysis as an advanced oxidation technology for removal of toluene from air using TiO₂ as a photocatalyst in building materials. First, the photocatalytic activity of two types of TiO₂ containing building materials, i.e. roofing tiles and corrugated sheets, has been investigated at ambient conditions (T=25.0 °C; relative humidity RH=47%; toluene inlet concentration [TOL]_in=17–35 ppbv). Toluene removal efficiencies up to 63% were observed at a gas residence time (τ) of 17 s. Second, the effect of RH (1–77%), [TOL]_in (23–465 ppmv) and τ (17–115 s) on toluene removal has been systematically investigated using TiO₂ containing roofing tiles as photocatalytic building materials. Results revealed lower toluene removal efficiencies at higher RH and [TOL]_in, whereas a positive effect was observed with increased τ. Under optimal conditions, toluene removal efficiencies up to 78±2% and elimination rates higher than 100 mg h⁻¹ m⁻² roofing tile were obtained. A decline in photocatalytic activity by a factor of 2 was observed after operation at gas residence times shorter than 69 s and [TOL]_in higher than 76 ppmv. Washing the building materials with deionized water, simulating rainfall, could partially (i.e. by a factor 1.3) regenerate the catalyst activity.     

Solar integrated energy system for a green building

Shanghai is characteristic of subtropical monsoonal climate with the mean annual temperature of 17.6 °C, and receives annual total radiation above 4470 MJ/m² with approximately 2000 h of sunshine. A solar energy system capable of heating, cooling, natural ventilation and hot water supply has been built in Shanghai Research Institute of Building Science. The system mainly contains 150 m² solar collector arrays, two adsorption chillers, floor radiation heating pipes, finned tube heat exchangers and a hot water storage tank of 2.5 m³ in volume. It is used for heating in winter, cooling in summer, natural ventilation in spring and autumn, hot water supply in all the year for 460 m² building area. The whole system is controlled by an industrial control computer and operates automatically. Under typical weather condition of Shanghai, it is found that the average heating capacity is up to 25.04 kW in winter, the average refrigerating output reaches 15.31 kW in summer and the solar-enhanced natural ventilation air flow rate doubles in transitional seasons. The experimental investigation validated the practical effective operation of the adsorption cooling-based air-conditioning system. After 1-year operation, it is confirmed that the solar system contributes 70% total energy of the involved space for the weather conditions of Shanghai.     

Design and performance of solar powered absorption cooling systems in office buildings

The paper contributes to the system design of solar thermal absorption chillers. A full simulation model was developed for absorption cooling systems, combined with a stratified storage tank, steady-state or dynamic collector model and hourly resolved building loads. The model was validated with experimental data from various solar cooling plants.As the absorption chillers can be operated at reduced generator temperatures under partial load conditions, the control strategy has a strong influence on the solar thermal system design and performance. It could be shown that buildings with the same maximum cooling load, but very different load time series, require collector areas varying by more than a factor 2 to achieve the same solar fraction. Depending on control strategy, recooling temperature levels, location and cooling load time series, between 1.7 and 3.6 m² vacuum tube collectors per kW cooling load are required to cover 80% of the cooling load.The cost analysis shows that Southern European locations with higher cooling energy demand lead to significantly lower costs. For long operation hours, cooling costs are around 200 € MWh⁻¹ and about 280 € MWh⁻¹ for buildings with lower internal gains and shorter cooling periods. For a Southern German climate, the costs are more than double.     

Wind effects of parapets on low buildings: Part 1. Basic aerodynamics and local loads

This is the first paper in a series on the effects of parapets on the wind-induced loads on low buildings. Part 1 focuses on the basic aerodynamic effects of parapets and the local (components and cladding) loads. Wind tunnel data were obtained from about 700 pressure taps in the area of a corner panel of 3.7 m×7.6 m (equivalent full-scale dimensions) for several parapet heights and configurations. Significant downward loads were observed which exceed code values for all parapet heights. This may be significant when combined with other loads (such as snow or water). It was also found that parapets alter the suction loads on the roof by changing the location of the corner vortex relative to the roof, for continuous perimetric parapets, and the type of vortex formed, for isolated (single wall) parapets. In the ASCE-defined interior region, the measured coefficients for component and cladding loads exceed those in the code for all parapets and areas examined. For the edge zone, the experimental coefficients for areas less than 1 m² exceed the code values (except for tall perimetric parapets). However, it was found that the component and cladding loads in the ASCE 7 adequately envelope the uplift caused by perimetric parapets in the corner zone for H=4.6 m, but not for isolated parapets, in particular for areas less than 1 m². It was also discussed that the ASCE 7 will be unconservative for larger eaves heights since H² is the correct normalizing factor for roof areas beneath the separated flow. Furthermore, the use of edge zone coefficients in the corner zone for h ?0.9 m should be changed to h/(H+h)?0.23 in the ASCE 7.     

Comparative environmental life cycle assessment of green roofs

This paper describes the life cycle environmental cost characteristics of intensive and extensive green roofs versus conventional roofs. A life cycle inventory and environmental impact assessment is used to document and analyze the similarities and differences in the environmental impacts of the fabrication, transportation, installation, operation, maintenance, and disposal of all three roof systems. This is important because there are additional resources committed to green roofs from which environmentally relevant benefits, such as reduced electrical energy use for building cooling, are derived. The extensive green roof design for the case study presented here is from an actual 1115 m² (12,000 ft²) green roof project on a retail store in Pittsburgh, PA, USA. The case study includes a conventional ballasted roof, an extensive, or shallow growing medium green roof, and an intensive, or deep growing medium green roof. For the life cycle inventory and the material use, both the types of material used and the transportation distances to the site are with respect to this project.     

Assessment of coconut fibre insulation characteristics and its use to modulate temperatures in concrete slabs with the aid of a finite element methodology

This paper presents the assessment of coconut fibre thermal characteristics and its use to modulate temperatures in concrete slabs in the construction industry. Fibre is abundantly available in tropical regions, extracted from the husk of coconut fruits and manufactured at 115.54 MPa to obtain specimens. A first thermal conductivity of k = 0.048 (W/m K) is obtained by solving the heat diffusion equation with experimental temperatures as boundary conditions. A second value k = 0.0499 (W/m K) is obtained by solving the Fourier's law by using a known heat flux and temperature histories in the specimen. The maximum error between the first and second k values was 3.8%. However, the k = 0.048 (W/m K) was used for numerical analysis.Experimental work was done to find density and heat capacity, 174 kg/m³ and 2600 J/kg K, respectively. Further numerical work was carried out to modulate temperature in concrete slabs. This showed that fibre put on the concrete external surface allows room temperatures to fall within the comfort range. Density, thermal conductivity and heat capacity of coconut fibre were varied in a wide range to investigate the sensitivity of temperature to such changes. This showed that temperature can be considered sensible only to thermal conductivity variations.     

Optimum insulation thicknesses for building walls with respect to cooling and heating degree-hours in the warmest zone of Turkey

Thermal insulation is one of the most effective energy conservation measures for cooling and heating in buildings. Therefore, determining and selecting the optimum thickness of insulation is the main subject of many engineering investigations. In this study, the determination of optimum insulation thickness on external walls of buildings is comparatively analyzed based on annual heating and cooling loads. The transmission loads, calculated by using measured long-term meteorological data for selected cities, are fed into an economic model (P₁−P₂ method) in order to determine the optimum insulation thickness. The degree-hours method that is the simplest and most intuitive way of estimating the annual energy consumption of a building is used in this study. The results show that the use of insulation in building walls with respect to cooling degree-hours is more significant for energy savings compared to heating degree-hours in Turkey's warmest zone. The optimum insulation thickness varies between 3.2 and 3.8 cm; the energy savings varies between 8.47 and 12.19 $/m²; and the payback period varies between 3.39 and 3.81 years depending on the cooling degree-hours. On the other hand, for heating load, insulation thickness varies between 1.6 and 2.7 cm, energy savings varies between 2.2 and 6.6 $/m², and payback periods vary between 4.15 and 5.47 years.     

Effect of swine manure dilution on ammonia, hydrogen sulfide, carbon dioxide, and sulfur dioxide releases

Animal manure is a significant source of environmental pollution and manure dilution in barn cleaning and slurry storage is a common practice in animal agriculture. The effect of swine manure dilution on releases of four pollutant gases was studied in a 30-day experiment using eight manure reactors divided into two groups. One group was treated with swine manure of 6.71% dry matter and another with manure diluted with water to 3.73% dry matter. Ammonia release from the diluted manure was 3.32 mg min⁻¹ m⁻² and was 71.0% of the 4.67 mg min⁻¹ m⁻² from the undiluted manure (P < 0.01). Because the ammonia release reduction ratio was lower than the manure dilution ratio, dilution could increase the total ammonia emissions from swine manure, especially in lagoons with large liquid surface areas. Carbon dioxide release of 87.3 mg min⁻¹ m⁻² from the diluted manure was 56.4% of the 154.8 mg min⁻¹ m⁻² from the undiluted manure (P < 0.01). Manure dry matter was an important factor for carbon dioxide release from manure. No differences were observed between the treatments (P > 0.05) for both hydrogen sulfide and sulfur dioxide releases. Therefore, dilution could also significantly increase the total releases of hydrogen sulfide and sulfur dioxide to the environment because dilution adds to the total manure volume and usually also increases the total gas release surface area.     

Key elements of and material performance targets for highly insulating window frames

The thermal performance of windows is important for energy efficient buildings. Windows typically account for about 30-50 percent of the transmission losses though the building envelope, even if their area fraction of the envelope is far less. The reason for this can be found by comparing the thermal transmittance (U-factor) of windows to the U-factor of their opaque counterparts (wall, roof and floor constructions). In well insulated buildings the U-factor of walls, roofs and floors can be between 0.1 and 0.2 W/(m² K). The best windows have U-factors of about 0.7-1.0. It is therefore obvious that the U-factor of windows needs to be reduced, even though looking at the whole energy balance for windows (i.e., solar gains minus transmission losses) makes the picture more complex.In high performance windows the frame design and material use are of utmost importance, as the frame performance is usually the limiting factor for reducing the total window U-factor further. This paper describes simulation studies analyzing the effects on frame and edge-of-glass U-factors of different surface emissivities as well as frame material and spacer conductivities. The goal of this work is to define material research targets for window frame components that will result in better frame thermal performance than is exhibited by the best products available on the market today.     

Power consumption benchmark for a semiconductor cleanroom facility system

Benchmarking is an important step in implementing energy conservation in a semiconductor fabrication plant (hereafter referred to as “fab”). A semiconductor cleanroom facility system is complicated, usually comprised of several sub-systems, such as a chilled water system, a make-up system, an exhaust air system, a compressed air system, a process cooling water (PCW) system, a nitrogen system, a vacuum system, and an ultra-pure water (UPW) system. It is a daunting task to allocate energy consumption and determine an optimum benchmark. This study aims to establish the energy benchmark of a typical 8-in. DRAM semiconductor fab through field measurement data. Results of the measured energy consumption index were: chilled water system (including chiller, chilled water pump and cooling tower): 0.257 kW/kW (=0.9 kW/RT) in summer and 0.245 kW/kW (=0.86 kW/RT) in winter air recirculation air system: 0.00018 kWh/m³ make-up air system: 0.0042 kWh/m³ general exhaust air system: 0.0007 kWh/m³ solvent exhaust air system: 0.0021 kWh/m³ acid exhaust air system: 0.0009 kWh/m³ alkaline exhaust air system: 0.0025 kWh/m³ nitrogen system: 0.2209 kWh/m³ compressed dry air system: 0.2250 kWh/m³ process cooling water system: 1.3535 kWh/m³ and ultra-pure water system: 9.5502 kWh/m³. These data can be used to assess the efficiency of different energy-saving schemes and as a good reference for factory authorities. The PCW system's status before and after implementing energy conservation is discussed.