EnergyPlus

EnergyPlus [49-50] is a comprehensive building energy simulation tool which is developed by the US Department of Energy. It is based on features from two separate packages – DOE-2 [51] and BLAST [52] – which were its predecessors. The main purpose of these two packages is the simulation of heating, ventilation and air conditioning (HVAC) systems and as a result, the core capability of EnergyPlus is a very mature HVAC systems simulation. Nonetheless, the package also has the capability to model electrical loads and generation including some complex analyses such as the yield of renewable energy systems at a particular location.

EnergyPlus is a simulation engine which is designed to accept its input and generate results through text files. Inputs to the system are a complete model of the building’s construction, definitions of the HVAC and electrical plant used in the building, and weather data for the building’s location. Results including plant utilisation, heat flows, room temperatures and total energy demand are provided in similar text files. The purpose of using text files rather than a full user interface is to allow third parties to develop their own interfaces that are

suited to their particular problem, and to allow EnergyPlus to be integrated into larger simulation systems. The Department of Energy develop a graphical interface for the package which allows building models to be rapidly developed in Google Sketchup [53] and simulated using EnergyPlus. This interface, known as OpenStudio [54], permits the whole process of defining the building model, its simulation and viewing the results to be carried out from Sketchup. Various architectural design software packages also provide their own interfaces to EnergyPlus [55].

Simulations within EnergyPlus are undertaken in two distinct steps. For each time-step in the simulation, a heat balance model calculates the temperatures of zones and surfaces within the building in addition to the heat flow between zones and surfaces. The results of these calculations are then fed into a building systems model which iteratively evaluates the response of the building systems to the changes in temperature that were calculated.

The heat balance model within EnergyPlus is a single-pass calculation rather than an iterative solver. The model takes the states of building zones and plant from the previous time-step of the simulation and uses these to compute the changes in temperature and heat flow for the current time-step. Two distinct modules are utilised for this purpose. The air mass module deals with heat flow due to moving air, either through forced ventilation or infiltration. This module assumes that in each zone of the building the air is of uniform temperature. The surface mass module deals with conduction, convection and radiation of heat from surfaces.

This module assumes that surfaces have uniform temperature, uniform radiated heat and one-dimensional conduction. Comprehensive coverage of the effects of windows on both heating and lighting of rooms is included in the heat balance model [56].

The building systems model is an iterative solver which represents the HVAC and electrical plant within a building. Building systems are simulated using a concept known as “loops”. A loop is a set of connected nodes which, in the case of HVAC systems, model heating and cooling plant connected by pipes or, in the case of electrical systems, model electrical generators and loads connected by wires [57]. At present two HVAC loop types are supported within the package – air loops and water loops. The iterative solver for HVAC loops defines loops with two halves – a demand side which provides the heating or cooling to

zones, and a supply side which provides the heated or cooled air or water. Temperature set-points are used for each zone to calculate the required heating or cooling input from its HVAC equipment. The solver then iteratively tries different configurations of all of the available plant within the room until the supply side of each HVAC loop can meet its demand side. Users can add control algorithms to this process to control the temperature set points of the rooms.

Electrical systems within EnergyPlus are also modelled as loops [58]. These loops can be either AC or DC electrical networks with a combination of loads, storage devices and generators. Conversion elements such as electronic inverters are included to bridge between DC and AC loops. There are a number of built-in generation modules including photovoltaic, wind, combined heat and power and internal combustion engines. Additionally, a number of control strategies for electrical systems are also included in the package which control the dispatch of generation and storage devices in response to both electricity demand and the target amount of power to be drawn from the grid. This allows for control strategies which encourage power consumption when grid prices are low and discourage consumption when prices are higher.

To conclude the study of EnergyPlus, consideration must be given to the accuracy of the results from its simulations. The package is tested using the ASHRAE standard 140-2007 (an update to ASHRAE 140-2001 [59]) which defines standard test scenarios for the benchmarking of building energy analysis software [60]. Results have been published which indicate that the results from EnergyPlus correlate well with those of other simulation packages [61-64]. In all of the tests presented in these results, the only situation in which the results from EnergyPlus fell significantly outwith the range of other simulation packages was in seven out of the thirteen building envelope tests where results varied by up to 15%

compared with other packages. In the HVAC and fuel-fire furnace tests, EnergyPlus produced results that were within 2% of those of other packages.