K.A. Antonopoulos

Experimental and theoretical studies of space cooling using ceiling-embedded piping

Abstract Space cooling is considered, in which the heat produced within or inserted into occupied spaces is transferred to a cold fluid flowing through piping embedded in the ceiling slab. Theoretical and experimental studies are conducted for the above transient thermal problem. The theoretical investigation is based on numerical and analytical solutions, while for the measurements an experimental set-up of physical size has been built up. A comparison of measurements and predictions shows that the theoretical procedures are reliable. A systematic parametric study is conducted and the ceiling slab cooling capacity (W/m 2 ), which is found to be the most important quantity for practical applications, is expressed as a function of the embedded pipe spacing and depth, and the temperatures of the inflow water and the indoor environment.

Performance of solar-driven ammonia-lithium nitrate and ammonia—sodium thiocyanate absorption systems operating as coolers or heat pumps in Athens

Abstract The hour-by-hour performance of solar-driven NH3=LiNO3 and NH3-NaSCN absorption systems operating as coolers or heat pumps in the Athens area is predicted, using 20 yr of local climatological data. The exact thermodynamic cycles are represented by substitution of the composite thermodynamic processes (absorption, generation, heat exchange), which involve interactions of two or three streams by thermodynamically equivalent one-stream changes. Under the assumptions made, it becomes possible to develop two groups of correlations. The first expresses the characteristics and the performance of the absorption systems in terms of the ambient temperature only, while in the second group the behaviour of the systems is expressed in terms of the hour of the day for each day of a typical year in Athens. The main conclusions for operation in the Athens area are: (a) For cooling purposes (summer) the maximum theoretical values of the coefficient of performance and of the cooling power are 90% and 355 W/m2, respectively, while for heating purposes (winter) the maximum theoretical values of the heat gain factor and of the useful thermal power are 210% and 344 W/m2, respectively. (b) For heating purposes (winter), the NH3-LiNO3 system is superior to the NH3-NaSCN one, because it provides a higher heat-gain factor and useful thermal power. (c) For cooling during summer, the choice depends on the special requirements of each application, because the NH3-LiNO3 system provides higher cooling power, while the NH3-NaSCN system achieves a higher coefficient of performance.

Improvements to the measurement of the thermal properties of phase change materials

Improvements are proposed to the well-known T-history method, which is widely used for thermal properties measurement of phase change materials (PCM). Our improvements refer to the experimental arrangement, to the way of measurement processing, as well as to the kind and presentation format of the final results. The proposed arrangement has a controlled indoor environment and is fully automatic, without need for staff attendance, even for repeated sets of measurements of the same or different PCM simultaneously. The proposed way of measurement processing is based on the use of thermal delay (i.e. temperature difference) between PCM and a reference fluid at any specified time and not in the use of their time delay at any specified temperature. This fundamental change leads to increased accuracy and considerable reduction of duration and labour of the measurement processing, as proved by the performed measurements of various PCM. The effective thermal capacity function as a final result is proved to be more useful than the results of the original method. The new procedure is a first step towards defining specifications for the measurement of PCM thermal properties.

Characteristics of the performance and emissions of a HSDI diesel engine running with cottonseed oil or its methyl ester and their blends with diesel fuel

An experimental study has been conducted to evaluate the use of various blends of cottonseed oil or its methyl ester (bio-diesel) with diesel fuel, in blend ratios from 10/90 up to 100/0, in a fully instrumented, four-stroke, High Speed Direct Injection (HSDI), Ricardo/Cussons Hydra diesel engine. The tests were conducted using each of the above fuel blends or neat fuels, with the engine working at a medium and a high load. Volumetric fuel consumption, exhaust smokiness and exhaust-regulated gas emissions such as nitrogen oxides, carbon monoxide and unburnt hydrocarbons were measured. The differences in the performance and exhaust emissions from the baseline operation of the engine, that is, when working with neat diesel fuel, were determined and compared, as well as the differences between cottonseed oil or its methyl ester and their blends. Theoretical aspects of diesel engine combustion were used to aid the correct interpretation of the engine behaviour.

Thermal parameter components of building envelope

A procedure is presented for analyzing the effective thermal capacitance, the time constant and the thermal delay of buildings into components corresponding to discrete sections of the envelope (i.e. the roof or a whole wall of a specified orientation), to envelope parts of different compositions (i.e. the brickwork and the concrete parts of the envelope), or even to the layers of the exterior multilayer walls. Correlations are also developed, which express the dynamic thermal parameters of buildings in terms of the thickness of exterior wall layers and the surface percentage of envelope parts with different compositions. The effective layer thickness is introduced, the increase of which causes negligible increase in the building thermal capacitance. The developed procedure is based on finite-difference solution of a rigorous set of coupled differential equations describing the dynamic thermal behaviour of buildings. The analysis made quantifies the thermal contribution of every element of the envelope and may improve its thermal behaviour if the related conclusions are taken into consideration in the design of buildings.

Effect of indoor mass on the time constant and thermal delay of buildings

A numerical procedure is presented for predicting the transient thermal behaviour of buildings and estimating time constant τ and thermal delay td. The proposed procedure is used to examine the effect of indoor mass on the above characteristics. The predicted effect of these is considerable; for example, the usual indoor mass in houses increases τ and td by up to 40 per cent, which is analysed to 25 per cent for interior partitions and 15 per cent for furnishings. Both τ and td are found to be linear functions of the mass of interior partitions Mp and furnishings Mf. These functions are given for typical buildings. A simple ‘time constant and thermal delay model’ is also proposed, which is in satisfactory agreement with the developed rigorous numerical solution. Components of τ and td are introduced, which express the contribution of any indoor construction element or piece of furniture to the ‘total’ time constant and thermal delay of buildings. The analysis in components, which are related to the corresponding ones of the ‘effective thermal capacitance’, provides useful information for the maximization of heat or cool storage in buildings. Copyright

Envelope and indoor thermal capacitance of buildings

Abstract The real or effective thermal capacitance of buildings quantifies the energy stored within and differs considerably from the apparent thermal capacitance, which results by adding distributed specific heats of building elements into a lumped value. In the present study, a method is developed for analyzing the total effective capacitance into components concerning the building envelope or parts of it (e.g. ceiling, floor, etc.), the interior partitions, the furnishings, etc. The developed procedure is based on a finite-difference solution of a set of differential equations describing the transient heat conduction in all elements of a building. Applications are made to 21 types of buildings with 15 and 10 wall and roof compositions, respectively, and floor area from 50xa0m 2 to 2500xa0m 2 . For example, it is found that for typical fully-insulated, one-storey, detached houses, the envelope, interior partitions and furnishings effective heat capacitances are 78.1%, 14.5% and 7.4%, respectively, of the total effective thermal capacitance. Also, a correlation is developed, which links the effective to the easily-calculated apparent thermal capacitance of buildings.

Experimental evaluation of energy savings in air-conditioning using metal ceiling panels

Abstract Space cooling using metal ceiling panels is analysed experimentally and theoretically. Measurements are performed in a 2.0×2.5×3xa0m 3 test chamber with a 1.80×2.16xa0m 2 cooling panel located on the undersurface of the chamber ceiling. Both experimental and theoretical analyses show that the dynamic response of the panel system in conjunction with the thermal comfort conditions is satisfactory for the climate of Greece. Under certain conditions the condensed water vapour may raise dripping problems, for which solutions are proposed. Energy savings exceeding 12.5% may be obtained from the increase in acceptable indoor temperature, but further savings are possible from the higher temperatures of cooling water, which improve the efficiency of solar-driven absorption chillers.

On the dynamic thermal behaviour of indoor spaces

Abstract A new model is presented for predicting the dynamic thermal response of indoor spaces to indoor heat pulses. The model is based on the concept of the “indoor surface thermal capacitance”, Cs, which characterizes the thermal inertia of an indoor space and expresses the heat stored within indoor air and surface layers of walls and furnishings, per degree of mean temperature difference between indoor air and building envelope. Extensive comparisons with measurements and rigorous finite-difference solutions show that the accuracy of the proposed model is satisfactory for a wide range of practical applications. Comparison with other indoor space simulations of the same class, characterized as “simplified approaches”, show that the present one may provide considerably increased accuracy.

Finite-difference prediction of transient indoor temperature and related correlation based on the building time constant

A numerical procedure for the calculation of the transient indoor temperature in buildings is developed. The procedure is based on an implicit finite-difference solution of a closed set of differential equations, which express the indoor energy balance and the transient heat conduction in all elements of the building envelope. Using the above procedure, calculations are carried out for 21 types of buildings with 18 and 10 different kinds of wall and roof constructions, respectively, i.e. the total number of building cases examined is 21 x 18 x 10 = 3780 with floor areas ranging from 30 m 2 to 3000 m 2 . It is found that buildings of different construction characteristics and sizes but with the same time constant, respond in a similar way under the same outdoor temperature variation. Based on this similarity, a correlation is developed which expresses, under periodic conditions, the indoor temperature variation in terms of the building time constant and the outdoor temperature characteristics. The correlation contains nine coefficients, the values of which are different for different ranges of the building time constant.