This paper deals with the modelling and simulation of small building power systems within the ESP-r simulation environment. The paper describes how an energy simulation tool (ESP-r) is run in conjunction with a power flow analysis solver to achieve solution of a thermal/electrical system.
Buildings, Combined Heat and Power, Energy Simulation, ESP-r, Power Flow Modelling, Photovoltaics.
The built environment is a complicated and many-faceted energy system encompassing many different yet synergistic energy flow paths. Each flow path changes its own (and hence the system's) characteristics. tics dynamically as time progresses. To accurately model such phenomena a simulation model must take into account all relevant energy subsystems and the interrelationships between them.
The areas covered by building simulation models are numerous and varied and include conduction modelling, air/fluid flow modelling, plant side modelling, insolation analysis, lighting studies and building climatic interaction. However one major area of energy usage and resulting financial expenditure is notably absent from this list and that is the ability to predict the electrical power consumption of the building.
The absence of adequate electrical power modelling within building simulators in general is an important omission when we consider that (in 1991) electricity accounted for 15% of energy consumption by end users, some 241,048 GWh, of this total some 155,655 GWh was consumed in the built environment $*$ , (58%). This accounts for some 8.7% of total energy end usage (DTI, 1992).In financial terms electricity sales to the residential, commercial and public sectors were 11,415m .
The built environment already accounts for some 19% of carbon dioxide emissions, however if we add on the figure for emissions due to production of electricity for the built environment this figure rises to around 39% of total CO2 emissions (61 million tonnes).
Analysing the figures it is apparent that consumption of electrical energy in the built environment is highly significant in both an environmental and economic context.
To date the majority of attention in the building simulation community has been focused on examining the energy end-use of the building system. This was largely due to the fact that the energy supply was seen as external to the system, electricity and gas were bought a large national supplier and so the objective was to minimise the usage of these energy sources. With the advent of combined heat and power, and laterly photovoltaic facades, energy production has become very much a part of the overall building system and hence the need to incorporate energy sources into the building model has become increasingly apparent.
ESP-r has been described as follows:
The theory behind computer modelling in power systems analysis has been well established for well over twenty years (Stagg and El-Abiad, 1965), while digital computers have been used in power systems simulation since the 1940's.
In general on power flow problems the solution method is by an admittance method . Normal methods of circuit analysis are not used as in most cases load impedances are not known, generally loads are known as complex powers while generation is modelled as a complex power source, rather than a voltage or current source which is the case in normal circuit analysis.
In power flow simulation the power system is modelled as a series of inter-connected
nodes, or buses with individual loads and generation sources connected
to a particular
bus. At each bus power is either generated, absorbed by a load or transmitted,
the
summation of these power flows is always zero. The purpose of the power
flow solution code
is to determine the power flow between nodes (of both real and reactive
power) plus the
voltages and phase angle at each bus bar. In power flow simulation a required
condition
for solution is that all loads are known, hence loads can be regarded as
boundary
conditions for the problem. Load calculation is the primary interface between
the building
simulation tool and load flow solution code with the simulation tool supplying
the power
flow analysis tool with dynamic load information at each simulation timestep.
In the ESP-r simulation environment a plant modeling capability can enable the simulation of generator output, hence the building or energy simulation program can supply all the required boundary conditions for the load flow analysis tool.
The main task of the load flow solution tool is to calculate the power flow between nodes. To achieve this a knowledge of the interconnections between nodes i.e. lines and transformers is required as well as a knowledge of the linking components admittance, (admittance is the inverse of complex impedance). The admittances are formed into a system matrix which, in essence contains all the information regarding inter-connections in the system.
In any type of power system there can be three types of buses encountered:
Load buses comprise over 80% of most systems, in the load bus real and reactive power flows are known but voltage and phase angle must be calculated.
The Generator bus is, as expected a bus to which a generator or multiple generators are linked, voltage and real power flow are regarded as known quantities, while reactive power and phase angle are unknown.
The important variables at each bus are P Gi, Q Gi, P Ti, Q Ti,PLi, Q LI and V i, representing the generated power , transmitted power, load power flows and voltage at the bus. The summation of the power flows is always zero. Solution is usually by means of an iterative method either Gauss-Siedel or Newton-Raphson, the two variables solved for each node are voltage and phase angle, from these values all other system properties can be determined.
Buses are linked by one of two components; the power transmission line or the regulating transformer. The transmission line is represented in the simulation by three values series reactance and shunt reactance. This representation contains the information on the line's resistive, reactive and capacitive properties. The purpose of the transmission lines is transfer power from one bus in the system to another. Like the transmission line the regulating transformer also transmits power from one point in the system to the other. The main purpose of the regulating transformer is to control the voltage of one of the buses it connects within defined limits. This is done by changing the impedance characteristics of the transformer with varying system conditions.
The power system equations are a set of 2n non-linear equations, where n is the number of buses. Due to the non-linearity of the system an iterative solution must be obtained. The two most common solution methods are Gauss-Siedel and Newton-Raphson. All values used in power systems solution are per unit i.e where the original value is a multiple of a user defined base value. An exhaustive explanation of the solution method is given in "Power Systems Analysis", (Gross, 1979)
Loads are modelled on ESP-r as individual components. If we consider the
example of a photocopier,
this component will create both a thermal and an electrical load on the
building. This load information
for each component or group of components is entered into the building
model. During the simulation
the total time-dependent electrical load for the building is calculated
and fed as a boundary condition to
the load flow solver.
6.2. Generation modelling on ESP-r
The type of generation found in modern energy efficient buildings is generally of a combined heat and power type. Examples of this are a gas engine unit with heat recovery powering a synchronous generator or a PV facade with heat recovery. These two distinct systems serve almost exactly the same purpose is both supplying power and thermal energy to the building.
Figure 7.1 Total Systems Modelling.
Solution of the overall system requires a simultaneous solution of both the building plant system and the electrical subsystem. The solution of the building plant system provides the boundary data for the electrical simulation. Information provided by the building side simulation is as follows;
Information provided by the power flow simulation is as follows;
The information from the load flow simulation is a snapshot of the system conditions at a particular instant and takes no account of transient phenomena with a small time constant.
The information derived from the system simulation can be used (in conjuction with a control strategy) to modulate system operation i.e. switching on and off loads in the building to reduce peak loadings, or switching on or off generation units to cope with changing demand.
Finally we will look at an actual simple system simulation carried out using the ESP-r power flow and building solver in parallel. The actual system modelled consisted of a 17-zone building with an array of PV panels on the roof. The PV was augmented with a grid connection. It is assumed that the PV power output is inverted and used to supply some of the building load. For information on the modelling of the PV material refer to "Modelling Active Building Elements with Special Materials" (Evans & Kelly, 1995).
Data for the PV panels is taken from the manufacturers data sheet, in this simulation the panels used are BP saturn panels with a peak output power of 354W per panel (Pulse test). The panel array is situated in the south facade and roof area of the building.
Figure 8.1 Simulation Model.
The simulation is run as an example of the modelling potential of combined
thermal/electrical simulation and hence the detail of the results output
is not
discussed here.
Figure 8.1 Sample Simulation Output.
Buresch M. Photovoltaic Energy Systems - Design and Installation , McGraw-Hill, New York, 1983.
Gross C.A., Power Systems Analysis , John Wiley and Sons, New York, 1979.
Clarke J.A, Energy Simulation in Buiding Design , Adam Hilliger, Bristol, 1985.
Stagg G.W, El-Abiad A.H., Computer Methods in Power Systems Analysis McGraw-Hill, New York, 1968.
Dept of Trade and Industry, Digest of United Kingdom Energy Statistics,
HMSO 1992
