Dennis L. Loveday

Convective heat transfer coefficients at a plane surface on a full-scale building facade

Abstract Accurate knowledge of the heat transfer processes at the external surfaces of buildings is necessary for design purposes. Using an experimental arrangement designed to provide measurements of good quality and accuracy, correlations are obtained for the external convection heat transfer coefficient hc as a function of wind speed for a plane, smooth test surface on the facade of an eight-storey building. Values for hc were correlated with wind speeds measured 1 m from the test surface and at 11 m above the roof. The correlations presented may be used for the prediction of hc values for the central region of smooth, multi-storey building facades between fourth and eighth storey levels inclusive.

The influence on building thermal behavior of the insulation/masonry distribution in a three-layered construction

Building envelopes are most often of a layered construction, the layers often consisting of insulation and masonry. This study investigates the influence of layer distribution and relative thickness of the insulation/masonry on the thermal behavior of the building and plant capacity. To evaluate this influence we have assumed that the overall thickness and the thermal transmittance (U value) of a three-layered building envelope are constant, but have varied the relative thickness and positions of masonry and insulation within the structure. Two structures were investigated: masonry/insulation/masonry, and insulation/masonry/insulation. The computer program BRE-ADMIT was employed; this program is based on the admittance procedure for analyzing building thermal behavior. Figures for the daily energy consumption in a single zone building together with the maximum power requirement are presented for the ranges of structures tested. It is shown that, for intermittent heating plant operation as opposed to intermittent cooling plant operation, the insulation/masonry/insulation structure saves 32–72% more energy compared with the masonry/insulation/masonry structure. For intermittent heating and cooling plant operation, a position can be found for the insulation layer within masonry/insulation/masonry structure, such that daily energy consumption is minimized but plant power requirement is maximized. Designers may find it helpful to be aware of these effects when designing buildings.

Displacement ventilation environments with chilled ceilings: thermal comfort design within the context of the BS EN ISO7730 versus adaptive debate

Abstract The current design standard BS EN ISO7730 [Moderate thermal environments—determination of the PMV and PPD indices and specification of the conditions for thermal comfort, International Standards Organisation (1995)] is based upon the work of Fanger, and essentially comprises a steady-state human heat balance model that leads to a prediction of the sensation of human thermal comfort for a given set of thermal conditions. The model was derived from laboratory-based measurements conducted in the mid-1960s in relatively ‘conventional’ environments. However, a chilled ceiling operated in combination with displacement ventilation represents a more sophisticated environment as compared with the original conditions in which the Fanger model was derived. This raised a question about the applicability of the current standard when designing for thermal comfort in offices equipped with chilled ceiling/displacement ventilation systems. This paper presents findings from an EPSRC-funded study that sought to answer the above question. Human test subjects (184 in total) carried out sedentary office-type work in a well-controlled environmental test room that simulated an office fitted with the above system. Measurements of environmental variables were taken at a number of locations near the subjects, each of whom wore a typical office clothing ensemble. The reported thermal comfort sensations were compared with values predicted from BS EN ISO7730 over a range of system operating conditions. It was shown that the current standard BS EN ISO7730 may be used, without modification, when designing for the thermal comfort of sedentary workers in offices equipped with chilled ceiling/displacement ventilation systems. These findings are interpreted within the context of a proposed modification to thermal comfort design standards that includes adaptive effects, and the influence of BS EN ISO7730 on the development of other radiant surface/displacement ventilation configurations is discussed.

Thermal comfort in chilled ceiling and displacement ventilation environments: vertical radiant temperature asymmetry effects

Abstract The paper presents some of the findings from a broader investigation aimed at determining thermal comfort design conditions for combined chilled ceiling/displacement ventilation environments. A typical chilled ceiling/displacement ventilation office has been created within a laboratory test room, in which the ceiling temperature can be varied over a range of typical operating values; the thermal comfort of eight female test subjects was then measured in the test room over the range of ceiling temperatures. Vertical radiant temperature asymmetry was found to have an insignificant effect on the overall thermal comfort of the seated occupants for the typical range of ceiling temperatures that would be encountered in practice in such combination environments. There was a slight trend for the reported sensation of ‘freshness’ to increase as ceiling temperature was reduced though this requires further study. It is concluded that existing guidance regarding toleration of radiant asymmetry is valid for thermal comfort design of chilled ceiling/displacement ventilation environments

Intelligence in buildings: the potential of advanced modelling

Abstract “Intelligence” in buildings usually implies facilities management via building automation systems (BAS). However, present-day commercial BAS adopt a rudimentary approach to data handling, control and fault detection, and there is much scope for improvement. This paper describes a model-based technique for raising the level of sophistication at which BAS currently operate. Using stochastic multivariable identification, models are derived which describe the behaviour of air temperature and relative humidity in a full-scale office zone equipped with a dedicated heating, ventilating and air-conditioning (HVAC) plant. The models are of good quality, giving prediction accuracies of ±0.25 °C in 19.2 °C and of ±0.6% rh in 53% rh when forecasting up to 15 minutes ahead. For forecasts up to 3 days ahead, accuracies are ±0.65 °C and ±1.25% rh, respectively. The utility of the models for facilities management is investigated. The “temperature model” was employed within a predictive on/off control strategy for the office zone, and was shown to substantially improve temperature regulation and to reduce energy consumption in comparison with conventional on/off control. Comparison of prediction accuracies for two different situations, that is, the office with and without furniture plus carpet, showed that some level of furnishing is essential during the commissioning phase if model-based control of relative humidity is contemplated. The prospects are assessed for wide-scale replication of the model-based technique, and it is shown that deterministic simulation has potential to be used as a means of initialising a model structure and hence of selecting the sensors for a BAS for any building at the design stage. It is concluded that advanced model-based methods offer significant promise for improving BAS performance, and that proving trials in full-scale everyday situations are now needed prior to commercial development and installation.

Investigating the Applicability of Different Thermal Comfort Models in Kuwait Classrooms Operated in Hybrid Air-Conditioning Mode

A field study to investigate the applicability of different thermal comfort indices was conducted in Kuwait classrooms operated in a hybrid air-conditioned mode, to assess thermal conditions during the school day. In Kuwait, the girls’ and boys’ schools are completely separated at all academic levels thus offering a wide range for investigating differences in thermal comfort sensations between both genders. A modified questionnaire was used to collect responses from lower age groups of pupils while thermal comfort variables were measured at the same time. However, this paper reports on findings for pupils in the age range 11-17 years. Data analyzed suggested that the different thermal comfort indices (PMV, ePMV, PMV10 and adaptive model and various comfort equations) under-predicted the students’ actual thermal comfort sensation on the warm side of neutral and over-predicted thermal sensation on the cool side of neutral on the ASHRAE comfort scale for both genders. Higher neutral temperatures were also predicted than those resulting from the students’ actual mean vote (AMV). A difference in neutral temperature was found between both genders.

External convection coefficients for framed rectangular elements on building facades

Abstract Using data obtained from a heated test plate fixed to the wall of an eight-storey building, correlations are presented for the external convection coefficient hc, as a function of windspeeds measured at 1 m from the test plate surface and at 11 m above the roof (in the free stream). The effects of a rectangular frame on the value of hc are measured for a range of frame depths, the geometries tested relating to facade configurations which are typical of glazing framework designs commonly found in UK commercial high-rise buildings and atria. For the stated conditions of use, the regression correlations may be used by designers and thermal modellers for the prediction of hc values for a range of typical building facades comprised of mullion and transom-type protrusions. The findings are particularly relevant to improving the accuracy of heat loss calculations from glazing.

The optimal design for a ground cooling tube in a hot, arid climate:

In many hot, arid climates there is a high level of use of domestic air conditioning which can make heavy demands on local electricity generation. A passive strategy which could mitigate this demand is the deployment of ground cooling tubes in which external air is drawn into the building/plant via a tube buried below ground. Whilst this principle has been known of for a considerable time, and many example installations described, there has been no systematic investigation of the influence of the design parameters on the viability of the system. The paper describes a method for carrying out a systematic parametric optimization for a ground cooling tube by coupling a validated numerical model of the system with a constrained optimization method using an evolutionary strategy. Optimizations were based on a thermo-dynamic objective function (minimizing external energy consumption of the whole system) and an economic function (payback time for the tube). Whilst the results are specific to the hot, arid climate of the state of Kuwait, the methodology has universal applicability and illustrates the functional power of combining a performance model with an optimization method in producing optimized designs of energy systems. Practical application: Building and plant simulation programs are becoming increasingly accepted as a design tool: for confirming the performance of a proposed design or for carrying out ‘what if?’ studies to evaluate the effects of varying design parameters or equipment selection. This paper illustrates that, provided the design problem can be modelled in all relevant aspects, linking the simulation program to a constrained optimization can provide significantly more decision support to the designer.

Nodal network and CFD simulation of airflow and heat transfer in double skin facades with blinds

The paper describes a modelling study of heat transfer and buoyancy-driven airflow in double skin facades consisting of a glass outer layer, a control device (venetian blind) and a double-glazed inner skin. The modelling study was based on two approaches — a component-based, lumped parameter simulation which used a public domain, open source differential/algebraic equation solver and a detailed, CFD calculation which included air flow, conduction, convection and radiation. The primary objective of the work was to compare the performance of the simplified model with the output of a rigorous CFD calculation.

Thermal performance of air-heating solar collectors with thick, poorly conducting absorber plates

Abstract Expressions are derived for the efficiency and loss factors of a coverless, air-heating solar collector in which it is possible to assign finite values for the thickness and thermal conductivity of the absorber plate. In the geometry treated, air flow is beneath a flat absorber, and heat transfer is both steady state and one dimensional. The expressions are validated by outdoor measurements from a fullscale tile roof used as a collector and by indoor measurements from tile and metal roof sections tested in a heating simulator. For the tile roofs, the expressions presented give better agreement with experiment than do expressions based on the usual “thin plate” models since the absorber is thick and poorly conducting. Efficiency characteristics of the full-scale tile roof are also presented. Trapezoidally profiled metal panels are commonly used as a cladding for industrial buildings; it is shown that their performance as collectors may be described by a “fin and tube” model. Information is presented in a format that is useful for design and may be used in computer models of the thermal performance of solar-assisted buildings comprising these, or similar, collection devices.