Question

Develop a computer program or spread sheet to calculate, for a given day and location, the hourly-changed total solar radiation onto a southeast-facing vertical wall and a flat-roof where solar panels are located. The day and location are user-defined. Show your results for May 21 in Newark, NJ.

Answer 1

Energy Estimation Methods The first step in calculating building energy consumption is to determine the loads on the building. Loads are instantaneous heat gains or losses that occur by conduction, convection, and radiation. Building loads can be summarized in the following categories: ? Conduction Through the Enclosure ? Infiltration ? Solar Heat Gain ? Internal Heat Gains ? Ventilation Heat transfer acting on a building at any instant in time are simple to calculate based on heat transfer physics. However, load calculations are complicated by thermal storage effects. Though a building experiences a certain heating or cooling load at any given time, energy is stored and released by thermal mass in the building, creating a time delay on the load experienced by the heating, ventilating and air conditioning (HVAC) system. Most energy modeling programs employ similar methods of calculating instantaneous loads but differ in how they account for thermal mass time delay effects. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE, 2007) has identified two primary methods of accounting for thermal mass in a building, the Heat Balance method (HB) and the Radiant Time Series method (RTS). There are also a number of older, lessaccurate methods that are no longer recommended by ASHRAE but still used by some modeling programs, including the bin method and the transfer function method. The HB method (ASHRAE 2007) is the most direct and accurate method of calculating heating and cooling loads, though it is complex and computationally intensive. In this method, a set of energy balance equations are created and solved for each surface in a building. (ASHRAE 2007). The HB method is described in detail in ASHRAE Fundamentals 2007. The programs EnergyPlus and ESP-r use the HB method.

The Transfer Function (TF) method (ASHRAE 2007) is an earlier version of the RTS method that is still used in common energy modeling software programs. This method applies weighting factors directly to all conduction and radiation loads without splitting loads into radiation and convection components. This method is very simple to apply and still accurate for annual energy calculations, though it is not accurate for peak or hourly load calculations. The program DOE-2 uses the TF method. The bin method (McQuiston 2005) is a very simple method of estimating annual building energy consumption. Weather data for a location is given in 5°F intervals or “bins”, with the number of hours of occurrence of each bin. It is assumed that the building uses the same amount of energy for each outdoor temperature bin. A load profile is developed to determine the building energy consumption for each bin. The bin method does not include hourly thermal mass effects. The program HOT2000 uses the bin method. 2.2 DOE-2 DOE-2 was funded primarily by the United States Department of Energy to provide a free energy modeling software program. DOE-2 has been developed by James J. Hirsch & Associates (JJH) and Lawrence Berkeley National Laboratory (LBNL). This program has been in use for more than 25 years. Many user interfaces have been created for use with the DOE-2 engine, including eQuest (James J. Hirsch & Associates 2009) and the Canadian-built EE4 (NRCan 2008). Figure 2-3 shows the structure of the DOE-2.2 simulation program (LBNL 2004). User inputs and weather data are used by the simulation engine in three stages: loads, HVAC and economics. The HVAC subprogram is further divided into systems and plant calculations. DOE-2.2 uses the transfer function method of calculating energy consumption. The engineering, physics and mathematics used in DOE-2.2 are documented in the DOE-2.1A Engineer’s Manual (LBL 1982), though this manual is out of date. There is no up-to-date, public engineering manual for this program and so it is sometimes difficult to understand the calculation methods used in the program