A Simple Approach for Building Cooling Load Estimation

: The present work is to demonstrate how some very simple problems are made mathematically complex and seemingly tedious due to academic or business compulsions. There are some problems in which mathematical model are developed after making many simplifying assumptions. But, when it comes to solving these models, very sophisticated and complex schemes are applied. For such problems, dual policy does not make sense and in many cases the problem may be tackled in a simpler way to get comparable accuracy. The present paper reports one such example. It deals with the development and authentication of computer software for estimating building cooling load. The software is simpler to use, needs fewer input data and is more versatile compared to any other commercially available, exorbitantly costly and extensively used software. The effects of significant building parameters like orientation, window glass shade type, number of glass panes used, wall insulation, roof type and floor type can be easily investigated. Effects of all these parameters have been investigated for a typical building block to arrive at an intelligent decision. With any other software or method, it cannot be made so conveniently. All the above mentioned advantages are without sacrificing accuracy and reliability.


INTRODUCTION
Building cooling load components are; direct solar radiation, transmission load, ventilation/infiltration load and internal load. Calculating all these loads individually and adding them up gives the estimate of total cooling load. The load, thus calculated, constitutes total sensible load. Normal practice is that, depending on the building type, certain percent of it is added to take care of latent load. Applying the laws of heat transfer and solar radiation makes load estimations. Step by step calculation procedure has been adequately reported in the literature [1][2][3][4] . It is a scientific and exact approach, but time consuming and lengthy. Overall heat transfer coefficients for all the components of building envelope are computed with the help of thermal properties of the building materials. For the design conditions and the building materials used, cooling load temperature difference, solar heat gain factors and cooling load factors are calculated. Principles of solar energy calculation are applied to determine the direct and indirect solar heating component of the building. The requisite data of building material properties, climate conditions and ventilation standard are also established and reported [5][6][7][8] . First principle is applied to yields the rates of heat transfer through different building components. All these components, when added up, give the total cooling (or heating) load of a building. This lengthy procedure makes the theoretical approach more of academic interest, which quite often, the design engineers do not prefer.
A widely popular method is by using load estimation forms; standard or developed by the designer/company. This approach saves both effort and time. Although it is an approximate method, it gives quite acceptable results for selecting suitable capacity of air conditioning units. The Air Conditioning and Refrigeration Institute (ACRI) load estimation form is very popularly used. There are many similar commercially available forms, which consist of tabulated data as function of design temperature difference. All the probable loads are included in these forms. These consist of direct solar radiation, transmission load through exposed walls (un-insulated and those with different degree of insulation), partition walls, all the possible types of walls, roof, ceilings, floors and outdoor air load. Some times, big companies prepare there own load estimate forms.
A third method is by applying computer software, standard and commercially available or developed by the designer/company. Due to omnipresence of personal computers, the third method remains the most popular these days.

Present approach:
The present authors first converted the ACRI load estimation form in SI units. In this load estimation form, all the transmission loads as well as ventilation/infiltration load are given as function of temperature difference between outdoor and indoor air. The factor F has been given for five arbitrary values of this temperature difference. For any given temperature difference the value of F is generally interpolated and multiplied with the area of the building envelope  Table 1.
With this new table, it was quite easy to write a computer program. The only data in tabular form to be incorporated into the program was that for direct solar radiation. The input data for this program consisted of the dimensions of all the conditioned spaces and the internal load components read from appropriate data tables. In each category of the building envelope, the type will be chosen by the program itself by an integer specified for a particular type in the conditional GO TO statement. The program can be executed in a single stretch to give printout of cooling loads of individual spaces, total sensible load of the building as well as it gross cooling load.

RESULTS AND DISCUSSION
First of all, the validity of the software was checked by making cooling load calculation for a typical building block shown in Fig. 1. The Carrier software was also used to calculate the cooling load for the same building. The building block had a height of 3 m and all the dimensions shown in Fig. 1 are in meters. The estimate by the present software was found to be 5.2% higher than that obtained from Carrier software. The present software has potential of investigating the effects of significant building parameters on the cooling load, which may be treated as its unique feature.

Fig. 1: A typical building block
The effect of building orientation, or the direction of transparent glass surfaces, has been shown in Table  2. The cooling load will be minimum when all the transparent surfaces are kept facing east. Load for other building orientations have, therefore been compared with this minimum load. Percent increase in each case has also been shown in this table. It is observed that the west facing glass surfaces result into maximum cooling load, which is 26.43 % higher than the minimum. The effect of shade type on the window glass is also investigated and has been shown in Table 3. As expected, the minimum cooling load was needed when there was outside awning. If no shading is provided, the gross cooling load will increase by 18.25%.  Table 4 shows the effect of type of window glass pane on the gross building cooling load. With single glass pane, the load is 8.9 % more than that with double glass pane windows. The effect of wall insulation has been shown below in Table 5. Without insulation, the gross cooling load was found to be 6.5 % higher than that when heavy 2 in insulation were used. This comparatively smaller increase necessitates having a comparative economic analysis before going for insulation of the walls.
The effect of roof type on the building cooling load was thoroughly investigated for all the roof types included in Tables 1. The findings of these investigations have been shown in the Table 6 shown below. Pitched flat roof with vented air space and 4 in insulation gives minimum gross cooling load. Flat roof with no air space and no insulation gives maximum gross cooling load, which is 80.8 % more than the minimum value. A large variation can be observed in the cooling loads with different types of roof. Special care is therefore, needed for designing the most suitable roof for an air conditioned building. The effect of floor type has been shown in Table 7. Floor over open crawl space gives 7.34 % higher cooling load than that over un-conditioned space.

CONCLUSION
The cooling load calculation described in the present paper is simply based on the rule of thumb. It may be called a computer version of cooling load estimation form. But surprisingly enough it gives very reliable results, which are almost the same as those obtained by the sophisticated and costly commercial software developed and marketed by the renowned MNCs. It is very easy to use and requires quite few number of data input. It is also capable of being used as a good tool to make thorough investigations of different building parameters and its orientation before starting the construction.