Sotar Energy Yol. 48, No. 4. pp. 227-233, 1992 0038--092X/92 $5.00+.00 Printed in the U.S.A. Copyright @ 1992 Pergamon Press Ltd. A NEW SIMPLIFIED METHOD THERMAL BEHAVIOUR PASSIVE SOLAR FOR EVALUATING THE OF DIRECT GAIN BUILDINGS A. C. OLIVEIRAand E. DE OLIVE|RA FERNANDES Department of Mechanical Engineering,University of Porto Codex, Porto, Rua Dos Bragas-40 99, Portugal Abstract--A new simplifiedmethod for calculating the monthly solar heating fraction of direct gain buildings is presented. Two different operating regimes have been considered: ( l ) thermostatically controlled tem- perature, typical of buildings with auxiliary heating systems; and (2) free-floating temperature, typical of buildings without auxiliary heating systems. In the latter case, to quantify the thermal behaviour of the building, a comfort solar fraction is introduced. In this method, the solar fraction is a function of three parameters: ( l ) the solar/load ratio; (2) the building thermal inertia; and (3) the monthly non-utilizability. Some examples of the application of the new method are presented, as well as comparisons with the results from existing methods. I. IN'IRODUCrION 2. THEORETICAl.BASIS OF ]'HE NEW METHOD In spite of the increasing facilities concerning the use of more powerful computers and software related to building energy simulations, the importance of sim- plified evaluation tools is still untouched. In fact, it is a general opinion that detailed simulations, hourly simulation programs, are not suitable for most of the possible users: they are time consuming and require a certain degree of specialization. A simplified method is intended as a low use-cost method that can be applied either by manual or small computational means. In comparison with detailed models, simplified methods have the advantage of needing much less input data: for instance, monthly averages of the climatic variables are used instead of hourly values. Simplified methods provide an indication (quan- tification) of the thermal performance of a building. They usually also allow the identification ofparameter groups that influence the building behaviour. Most of the existent simplified methods are corre- lation based[ 1,2], obtained through simulations per- formed for a variety of system parameters. However, their applicability is restricted by the functional form (parameters) used in the correlations. Some methods were developed for particular climates, building con- struction characteristics, or building operating strate- gies. Efforts must be made toward the development of a more general method. The building thermal inertia must be quantified in an adequate way. In the case of buildings that operate without auxiliary heating sys- tems, it would be important to quantify the degree of thermal comfort. This case is typical of residential buildings: in Mediterranean countries, for instance, most of them operate this way all year round, and in colder climates buildings sometimes operate in this way during the warmer months of the heating season. 2.1 Thermostatic control We will start by considering a building with an aux- iliary heating system activated when the air tempera- ture reaches a minimum value or set-point. Figure 1 shows the heat fluxes involved, taken for a long period of time (one month). The solar gains (Q~o~)are equal to the solar energy transmitted through the glazing(s) and absorbed by the internal surfaces. The losses through the building envelope can be expressed as the sum of two terms: the reference heating load (Q=f), which is the energy that the building would require if there were no solar gains, and the excess energy (Qe~¢), which corresponds to the increase in the temperatures inside the building (air and surfaces) due to the solar gains. In a real building, the instantaneous solar gains are either used to supply part ofthe losses, or stored in the building structure for later use. Some part of the solar gains are not useful, since they overheat the air, i.e., above the set-point temperature, Train.The fraction of solar gains that are useful to the reference load, divided by the load, is called the solar fraction: f = O~,,.oUO~of ( 1 ) which, using the heat balance suggested in Fig. 1, can be written as f= I - Qa.,IQ,~r (2) since Q~,.,s = Q ~ . - Qe,<. (3) Until now we have not considered the internal gains of the building. They can be included indirectly in Q,a, since they contribute to reducing the heating load. The 227