In this work, a computer program, VIETSIM, was written specifically for Vietnamese conditions. This approach was taken for a number of reasons. First, although the TRNSYS program is well-known and used worldwide, there are problems in applying the package directly to Vietnamese conditions. The method for generating synthetic hourly solar radiation and ambient temperature sequences, which the TRNSYS Weather Generator Subroutine uses, is not likely to be suitable for tropical countries like Vietnam, as indicated in previous studies. In this work, the submodel for generating weather data uses a different approach, and can more accurately generate hourly solar radiation and ambient temperature sequences for Vietnam. Second, with the current version of TRNSYS 14.1, users still need to have a certain background knowledge of FORTRAN programming, as recommended in TRNSYS itself, which is not readily available in Vietnam. The VIETSIM program, in contrast, is more user
friendly, and will be interfaced into the windows environment for the convenience of users. Third, TRNSYS is a package capable of handling many different solar thermal systems, whereas VIETSIM is targeted at the single solar thermal application, namely water heating.

There are three main submodels in the VIETSIM program. The first is used to generate hourly solar radiation and ambient temperature data, the two main weather variables for SHWS simulation, from monthly mean daily solar radiation, or monthly mean sunshine hours, and monthly mean ambient temperature. The second is the program for simulating SHW systems; and the third is the submodel for undertaking an economic analysis of the particular application.

The quantity integrator function is used calculate and display hourly, daily, weekly, monthly and/or yearly values of system performance, as users desire. Table 1 shows an example of monthly outputs from this submodel. This submodel has been validated by comparing its results with those from TRNSYS. The weather file for Hanoi, generated from submodel 1 of VIETSIM, was used to run TRNSYS. There is a small difference in the total energy supplied by the tank between the two programs. This difference results from the use of different approaches to calculate the tank energy: TRNSYS used the plug flow algebraic model; whereas VIETSIM is based on the multinode differential equation model. However, the difference is small and can be ignored.

( All energy is in GJ )

An easy to use user interface in the Windows environment using Borland Delphi has also been developed. It is aimed at providing a user with a friendly environment for the selection of different simulation and design options. The user can use VIETSIM for both simulation and design purposes as desired. The weather data can be entered as hourly values or simply as monthly average values, or even monthly average sunshine duration. Furthermore, the user can directly use the solar contour maps by moving the cursor to the location needing to be investigated in the maps.

The program will pick up latitude, longitude, and monthly average daily solar radiation and then generate hourly solar radiation sequence in order to enter to VIETSIM as input data. The VIETSIM model calculates values for the key parameter required for system economic analysis, namely the solar fraction. Figure 7 shows the interface window of the overall VIETSIM program. To upgrade the interface, several in-built functions and graphical display features may be easily incorporated in the future.