Technical Overview
LCR Methodology
A concentrated effort of research, design, construction, monitoring, and evaluation of actual buildings started at the First Passive Solar Conference in Albuquerque, NM in 1976, and continued into the late 1980’s. It is estimated that more than 200,000 passive solar homes were built in the United States during this time. This wealth of experience was reviewed by the National Renewable Energy Laboratory, the Technical Committee of the Passive Solar Industries Council and, by the Standing Committee on Energy of the National Association of Home Builders. This review effort was distilled into the Passive Solar Design Strategies: Guidelines for Home Building.
BGW2004 Analysis
The analysis procedures used throughout BGW were developed using simple, well established methods for estimating the performance of passive solar heating and natural cooling strategies. These procedures were originally developed at the Los Alamos National Laboratory with funding from the U.S. Department of Energy Solar Buildings Program.
Heating Conservation Performance
The heat loss calculation procedure is based on a summation of the building envelope components heat loss coefficients (excluding the solar components). Annual heat loss is estimated by multiplying the heat-loss coefficient by annual degree days times 24 to convert from days to hours.
Degree days for each month were determined using an appropriate base temperature that accounts for an assumed thermostat setting of 70 degrees, an assumed internal heat generation of 36 Btu/day per sq. ft. of floor area and, the building loss coefficient. These form the basis for a heating degree day multiplier which is applied to the location heating degree days.
The conservation performance is an estimate of the annual heat required to maintain comfort, excluding both positive and negative effects resulting from the solar components. In this estimate no solar heating credit is given to east, west, and north windows, because it is assumed that these will be protected by vegetation or other shading. This is a conservative assumption because there will always be some solar gain through these windows.
Heating Auxiliary Performance
Passive solar savings fractions are calculated using the solar load ratio (SLR) method. Monthly solar savings fraction (SSF) values are determined using correlation fits to the results of hourly computer simulation calculations for a variety of climates. These 12 values are converted into an annual value used in the BGW calculations. The SLR method gives answers that agree within about 5% of the hourly computer simulations and within 11% of the measured passive solar performance of 55 buildings monitored under the Solar Buildings Program. The SSF estimates account properly for both solar gains and heat losses through the solar aperture and, thus, correct for omitting the solar components from the calculation of annual heat loss (Conservation Performance).
Auxiliary Cooling Performance
The cooling analysis method is based on a modified monthly degree-day procedure in which the day is divided into day and night periods2. All estimates are derived from correlation’s based on hourly computer simulations. Solar, conduction and, internal gains are estimated for each half-day period in each month. Delay factors are used to account for heat carry over from day to night and night to day. The results are estimated annual sensible cooling delivered by the air conditioner and do not include latent loads.
The original Los Alamos monthly procedure has been simplified in the WinGuide calculation procedure. Heat Gain Factors and Internal Gain Factors are annual incremental cooling loads resulting from a one-unit incremental change in the respective heat input parameter (that is, a one-unit change in UA, glazing area, or number of bedrooms). The combined heat load resulting from all inputs is summed and then adjusted for thermal mass and ventilation. This correction includes a constant required to match the calculated cooling load of a base case building (similar to a MEC Standard Design). This linearized procedure gives accurate estimates for cooling loads that are less than about 150% of the MEC Standard Design; however, it underestimates very large cooling loads in poorly designed buildings.
The adjustment factors for ventilation properly account for maintaining comfort in hot and humid climates. Ventilation is restricted to times when the outside dew-point temperature is less than 62ºF. This restriction avoids ventilation when high humidity might cause discomfort.
Temperature Swing
The temperature swing estimate on the performance card is based on the diurnal heat capacity method 3. The method is an analytic procedure in which the total heat stored in the building during one day is estimated by summing the effective heat storage potential of all the various materials in the building for a 24-hour periodic cycle of solar input. Rooms with direct gain are assumed to have radiative coupling of the solar heat to the mass. Rooms connected to rooms with direct gain are assumed to have convective coupling, which is rather less effective, especially for massive elements. The dhc of the sheetrock, framing and furniture approximated as 4.5 or 4.7 Btu / ºF / sq. ft. of floor area. The Mass Card provides a list of materials and their diurnal heat capacity which can be used to provide increased heat storage capacity.