Impact of simultaneous simulation of buildings and mechanical systems in heat balance based energy analysis programs on system response and control

The current generation of building simulation software is based upon separate building, mechanical system, and equipment simulations. This scheme evolved primarily because of memory limitations of the computers which were used to develop the programs. Hardware advancements have eliminated some of these limitations so the separate building and system scheme needs to be reevaluated. In addition to discussing methods of introducing simultaneous building and system simulations into the BLAST program, this paper will also address new system specification and control strategy options which are made possible by the simultaneous simulation method. BLAST currently uses a linear univariate control profile to describe the heating and cooling provided by the fan system as a function of room temperature during the loads calculation. Control profiles for each thermal zone are used to model the system response during the loads simulation. Alternatively, a combined simulation of the zone and the system determines the system output by allowing each system component to respond to changes within the zone and the outside environment. The combined simulation technique also allows modelling of systems which are impossible to represent using BLAST control profiles: for example, cooling to the zone provided by outside air ventilation. The combined simulation is accomplished by using time steps short compared with conventional hourly energy-balance based programs, but long compared with finite difference methods. In addition, the system response is allowed to lag the zone conditions by one time step. This completely eliminates iteration from the solution procedure: however, instabilities may be introduced due to the feedback between zone and system. Methods have been developed to simulate physical controllers which modify the response of system components to variable zone conditioning demands. The use of short time steps also affects the calculation of conduction transfer functions (CTF's) used to compute surface temperatures and heat fluxes. For a given construction the accuracy of CTF calculations decreases as the number of terms in the CTF series increases, due to round-off and truncation errors. This problem has been avoided by calculating the CTF series at large enough time steps to ensure accuracy and maintaining a "master" set of surface temperature and flux histories from which intermediate "temporary" histories can be interpolated to give temperatures and fluxes at the desired times. This paper specifically discusses the results of performing a complete system simulation within the loads calculation portion of the BLAST program by using a shortened time step combined with lagging the system simulation. In addition, new ideas for specifying systems to take advantage of this scheme are presented along with concepts for system control.