F.D. Heidt

Real heat recovery with air handling units

Abstract More and more air handling units are equipped with heat recovery systems, with the aim of decreasing the energy use in buildings for heating and cooling. The efficiency of the heat recovery system is often used to calculate the energy saving. However, air-handling units do not always function as planned. In particular, parasitic shortcuts and leakage may decrease dramatically the efficiency of ventilation and heat recovery. In addition, these units need electrical energy for fans, which may be more precious than saved heat. Measurements, using tracer gas dilution technique have detected various malfunctions in several units. This paper addresses real energy recovery with air handling units from a theoretical point of view and presents results of measurements on 13 units. In the best three cases, the real, global heat recovery efficiency was between 60 and 70% for units having a 80% nominal efficiency. In the three worst cases, the global efficiency was less than 10%. For these cases, the heat recovery system uses more energy than it saves.

Evaluation of the cost efficiency of an energy efficient building

This paper presents quantitative results on the economics of different levels of thermal insulation for a building envelope. Results are calculated on the basis of an example building in Germany, which is a recently built single family house with materials commonly used in light-weight constructions. Defining the cost efficiency of an energy efficient building allows one to identify solutions which are already economically viable as well as to determine specific costs of the investment in an advanced sustainable building.

The growth of the mixed layer in a stratified fluid due to penetrative convection

This paper describes a theoretical and experimental study of penetrative convection within an initially thermally stably stratified fluid heated from below. Emphasis is placed on the experimental investigation of the growth of the mixed layer and the entrainment at its boundary. Both processes play an important role in density-induced geophysical phenomena such as the lifting of an inversion layer during the morning and the deepening of a thermocline in a lake during the fall.Many laboratory experiments with water as the experimental fluid were performed, in which the convection process was generated and visualised. The height of the mixed layer, heat transfer across the bottom interface and temperature profiles were measured as functions of time.Theoretically-based analytical equations are given, which predict the thickness and temperature of the mixed layer. The equations involve one empirical factor characterising the entrainment rate at the interface between the mixed and the upper stable layer.The experimental results confirm the theoretical equations and show that the empirical factor is a constant. From this, an entrainment rate is calculated which agrees well with values presented in the meteorological literature.

SOMBRERO : A PC-tool to calculate shadows on arbitrarily oriented surfaces

Shaded areas of windows, solar thermal collectors or photovoltaic modules are of major importance for the calculation of solar heating- and cooling-loads of buildings as well as for the determination of thermal or electrical output of corresponding solar equipment. SOMBRERO, a PC-program written in Turbo-Pascal, calculates the GSC (geometrical shading coefficient), the proportion of shaded area of an arbitrarily oriented surface surrounded by shading elements as a function of time and location. Shading elements are treated as polygons (not necessarily rectangles) located in a plane and can be combined to bodies such as buildings or trees. They may also represent overhangs and side-wings of the building under consideration. Elements which are far away from the receiver area are treated as horizontal shading profiles. The reduction of (isotropic) diffuse radiation caused by different kinds of obstacles is calculated by means of view-factors. Calculated results of the GSC are stored in ASCII-format and can be used as an input for dynamic solar system simulation programs. This is demonstrated in two illustrative examples showing the coupling of SOMBRERO with SUNCODE and TRNSYS in order to calculate the effects of shading on passive solar heating and passive cooling, respectively.

Microcomputer-aided measurement of air change rates

Abstract Air change rates are measured by a non-dispersive one-beam IR gas analyzer using the decay and constant-emission methods with nitrous oxide as tracer gas. Disturbing influences due to H 2 O and CO 2 are low. The analyzer is coupled via a RS-232-C interface to a microcomputer, which is programmed to service the following functions: (1) calibration, (2) preparation and control of measurements, (3) recording, displaying and storing of data, (4) evaluation of results, and (5) error analysis. The implemented programs provide an instant access to results. The whole equipment is installed in compact form on a mobile rack. Measurements have been taken in a university laboratory to examine air change rates with (1) closed door and window, (2) open door only, and (3) tilted window only. Typical results are given and show where the decay method or the constant-emission method is more appropriate.

Mapping of global radiation with meteosat

Abstract For detailed mapping of global radiation received at ground the Heliosat method of Ecole Nationale Superieure des Mines de Paris, Sophia-Antipolis (France), is applied. Preliminary results of a validation for Germany are presented. Precision of Heliosat insolation estimates is assessed by comparison of monthly averaged hourly and daily sums and of weekly sums acquired in August 1994 and February 1995 with pyranometric measurements from 4 stations of the German Weather Service (DWD). The predictions RMS error amounts to 7% of the observed August 1994 monthly averaged daily global irradiation of 4285 Wh/m2 and to 15% of the observed February 1995 monthly averaged daily global irradiation of 1297 Wh/m2. The error of the estimation of monthly averaged hourly global irradiation is less than 100 Wh/m2. These values are within the limits of what has been reported hitherto.

STEADY STATE HEAT BALANCE AND SURFACE TEMPERATURE OF SOLAR RADIATED INDOOR WALLS

ABSTRACT Measurements on a laboratory model permit the verification of the heat flow balance on the internal wall of a cubic test box. The extensive measurement evaluation and complex data analysis is carried out with a microcomputer. As a result of the analysis, the parameters are set for a mathematical model which excellently reflects the measurement results. The net heat flow of an internal wall surface can now be defined in terms of the proportion of solar, infrared and convective components under known boundary conditions. Convection can be described by means of the Nu(Ra) relation usually used for turbulence, and the influence of the temperature field on the flow pattern is recorded by means of two different-sized parameters Nu/(Ra1/3). In general, the amount of convective flow is smaller than that of the infrared flow. Multiple infrared reflections must not be neglected. The customary linearized models used in building physics will have to be modified accordingly. Models with constant generalized surface conductance α* and hypothetical outside temperature T* are of only limited worth, as the calculated value of α* has to be worked out numerically and cannot generally be set in advance.

Radiative and convective heat exchange inside a test box with solar gain

Abstract Measurements on a laboratory model permit the verification of the heat flow balance on the internal wall of a cubic test box. The extensive measurement evaluation and complex data analysis are carried out with a microcomputer. As a result of the analysis, the parameters are set for a mathematical model which excellently reflects the measurement results. The net heat flow of an internal wall surface can now be defined in terms of the proportion of solar, infrared and convective components under known boundary conditions. In general, the amount of convective flow is smaller than that of the IR or solar flow. Multiple IR reflections must be accounted for. The customary linearized models used in building physics for external wall surfaces will have to be modified for internal walls accordingly. Convection can be described by means of the Nu(Ra) relation usually used for turbulence, and the influence of the temperature field on the flow pattern, is recorded by means of two different-sized parameters Nu/(Ra 1 3 ) . From dimensional analysis, scaling rules are worked out in order to interpret data obtained from experiments on laboratory models for full-sized conditions in nature and vice versa .

IDEA: Interactive Database for Energy‐efficient Architecture

The project IDEA – Interactive Database for Energy‐efficient Architecture – is a multinational collaborative project to build a European knowledge base on advanced energy conscious building design. At the core of this project are two earlier multimedia developments, the Swiss program DIAS and the German adaptation NESA which present the principles and applications of solar architecture in the respective national contexts. IDEA aims to improve know‐how of European professionals and exchange of information between them. The database includes information on exemplary buildings, their technical concepts and on other specific conditions. IDEA presents the existing state of the art in the field of energy‐efficient architecture by means of a multimedia database of built examples, an encyclopedia of design concepts and a set of interactive design support tools. The built examples have been selected from around Europe. IDEA assembles technical information on materials and energy‐related standards and regulations and provides climatic data from all relevant European regions.

HEAT RECOVERY FROM WASTE WATER BY MEANS OF A RECUPERATIVE HEAT EXCHANGER AND A HEAT PUMP

ABSTRACT The useful heat of warm waste water is generally transferred to cold water using a recuperative heat exchanger. Depending on its design, the heat exchanger is able to utilise up to 90% of the waste heat potential available. The electric energy needed to operate such a system is more than compensated for by an approximately 50-fold gain of useful heat. To increase substantially the waste heat potential available and the amount of heat recovered, the system for recuperative heat exchange can be complemented by a heat pump. Such a heat recovery system on the basis of waste water is being operated in a public indoor swimming pool. Here the recuperative heat exchanger accounts for about 60%, the heat pump for about 40% of the toal heat reclaimed. The system consumes only 1 kWh of electric energy to supply 8 kWh of useful heat. In this way the useful heat of 8 kWh is compensated for by the low consumption of primary energy of 2.8 kWh. Due to the installation of an automatic cleaning device, the heat transfer surfaces on the waste water side avoid deposits so that the troublesome maintenance work required in other cases on the heat exchangers is not required.