Why do we need energy modelling and what are the big issues?
- To conform with legislative requirements.
- To attain a requisite levels of comfort.
- To attain indoor air quality.
- To embody renewable energy technology.
- To improve energy efficiency.
- To lessen environmental impact.
What are the 4 main challenges associated with modelling energy systems?
- They are STOCHASTIC - ie wind direction and speed can be assumed to follow a trend but at certain instances there is no way to know what will happen.
- They are NONLINEAR - ie the defining material parameters depend on the variables of state (temp. moisture content) which cannot be found without first knowing the parameters.
- They are SYSTEMIC - the flow paths are all connected and influence each other.
- They are DYNAMIC - different flow paths vary at different rates at different times.
What are the types of modelling used to model energy systems?
- ANALYTICAL METHOD - these can be configured in time or frequency. The Admittance method is an example that is configured in the frequency domain.
- NUMERICAL METHOD - Conservation equations are formed for each individual finite element/finite volume. These elements represent the discretised problem and the equations for all of them are solved simultaneously.
Why are simulations necessary?
- The analytical method is poor and requires that the boundary conditions are approximated and systemic effects to be neglected. (This means BCs are assumed to be cyclic or impulses.)
- Analytical models are insufficient at analysing things such as mould growth.
- Complex transient problems don’t work in analytical models and simulations make these a lot easier.
- variable relationships are nonlinear.
- Variables interact with each other (systemic).
List the flow paths and state their units.
- TRANSIENT CONDUCTION - k = W/mK, rho = kg/m^3, C = J/kgK
- SURFACE CONVECTION - h = W/m^2K, deltaT = K, A = m^2
- INTERNAL LONG-WAVE RADIATION EXCHANGE - deltaT = K, emissivity = NA, View Factor = NA, Surface Reflection (grayness) = NA
- EXTERNAL LONG-WAVE RADIATION EXCHANGE - Tsurface = K, Cloud Cover and Type = %, emissivity = NA
- SHORT WAVE RADIATION - absorptivity = NA, reflectivity = NA, transmissivity = NA, angle of incidence = degrees, amount of irradiance = W/m^2
- AIR FLOW - Infiltration, Zone-Coupled Flow and Mechanical Ventilation.
- CASUAL GAINS - heat gains from lighting, occupants = W
- CONTROL - sensor > response > actuator.
- MOISTURE - fluctuations in moisture levels.
Why do the energy flow path parameters need to be evaluated at each time step?
- SYSTEMIC EFFECTS - the properties change at each time step as they depend on the variables of state, which change at each time step.
- TRANSIENCE - if the temperature of the surface at each time step changes then the amount of radiation leaving that surface will change.
What are the weather parameters and their units?
- DRY BULB TEMPERATURE = degrees celcius
- WET BULB TEMPERATURE = degrees celcius
- GLOBAL HORIZONTAL SOLAR RADIATION = W/m^2
- DIFFUSE HORIZONTAL SOLAR RADIATION = W/m^2
- NET LONGWAVE RADIATION = W/m^2
- WIND SPEED = m/s
- WIND DIRECTION = degrees from north
- PRECIPITATION = mm
- ATMOSPHERIC PRESSURE = bar
- CLOUD COVER AND TYPE = 5
- SUNSHINE HOURS = hr
What is the Response Function Method?
Provides and analytical solution to to differential equations that govern the flow of heat in solids, heat transfer at surface layers and heat exchange between connected fluid volumes (ie water to air).
RFM usually used in low-order, time invariant problems.
RFM is often used to estimate the internal room temperature in an unconditioned building. Can be used in reverse to calculate the conditioning requirements to keep a building at a certain temperature.
The frequency domain RFM is known as the Admittance Method.
What are the steps in the Response Function Method?
For the time domain:
- Heat transfer equations in time domain is transferred to imaginary space using Laplace Transforms.
- This imaginary (subsidiary) equation is solved algebraically for a unit boundary condition to give a Unit Response Function (URF).
- The actual BCs are approximated as triangular approximations.
- The BC approximation is multiplied by the URF to give the system response to this BC.
- Responses from the different combinations of BCs/URFs are added together to give the overall system response.
What is the difference between the time and frequency domains?
- Time domain: This operates in the time domain and approximates continuous data such as heat flux and temperature as a series of triangular pulses.
- Frequency domain: In frequency domain and approximates continuous phenomena as a series of sine waves of increasing frequency and reducing amplitude. This allows a unit boundary condition to be applied in the form of a square wave.
Define and Compare Solar Temperature and Environmental Temperature.
Sol-air Temperature: Is the fictitious temperature that would cause the same amount of heat transfer to a surface/building, in the absence of all heat transfer from the sun that the sun causes.
Environmental Temperature: The actual temperature of the environment that is being analysed.
What are the steps involved in making a numerical model?
- System is discretised into finite volumes to represent parts of the system.
- Conservation equations are developed for each inter-FV connection and transfer type.
- The equation set is solved simultaneously for successive time steps under evolving boundary conditions.
What approximations are made to represent the PDEs for the problem?
- DISCRETISATION: splitting a continuous system into sections will have an error involved as the system is not in sections in reality. The smaller these finite volumes, the more accurate the system is represented.
- TRUNCATION ERROR: This is the error involved in approximating an infinite sum as a finite sum. For example, the conservation equations will be infinite but they are approximated to be finite.
Explain the Simple and Detailed discretisation schemes.
SIMPLE (sequential): There are only a few nodes for the whole system. This would give the overall consumption and some performance estimates. Does not give any detail on how the parameters vary within the components.
DETAILED (simultaneous): This is a system where there are a lot of nodes present. The system can show the overall performance but the rates of changes of parameters can also be seen at different positions as there are many nodes for each component. This can be used to see where there is a loss in efficiency etc.
Once an energy system has been simulated, what are the two ways in which it can be solved?
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Summarise the steps involved in estimating the total shortwave irradiance on an inclined plane.
- Find the incidence angle (i beta)
- Find the sky diffuse irradiance (I sbeta) - this is normally an equation that is provided.
- Find the direct incident radiation (I dbeta) - this equation is not given, we must remember it.
- Find the ground reflected radiation (I rbeta) - this equation is not given and must be remembered.
- Find the total irradiation on the tilted surface (I Tbeta) - this is just the sum of the previous 3 irradiances.
Summarise the steps involved in the numerical models for long-wave radiation.
- Surface divided into finite elements and a unit hemisphere applied to each one.
- Surface of the unit hemisphere is divided into patches. These patches represent the radiosity of the finite element (the total radiation leaving the finite element).
- ‘Energy rays’ are formed by drawing a line form the centre of the finite element to the centre of the surface patch.
- The ‘energy rays’ are projected to find which other surfaces they intersect.
- At the intersection, a surface model is created. This allows the amount of incident radiation and radiation leaving the surface to be summated.
- This ray processing ends when the inherent energy on that surface falls below a threshold level.
- The energy absorptions are then summated for each finite element and summated to give the final long-wave radiation exchange for the enclosure.
What are the steps involved in the nodal network method?
- BCs are represented by temperature and wind velocity.
- The system is represented by a series of nodes where the pressure is either known or unknown.
- Buoyancy effects included by modifying the nodal fluid densities as a function of node temperature.
- Empirical flow relations applied to determine the flow within the components as a function of pressure differences.
- Iterate to solve the mass balance at each node to find the pressure at each node.
What are the steps involved in the CFD method for fluid flow problem?
- Discretise the domain.
- Apply the conservation equations to the finite volumes.
- Apply the initial and boundary conditions.
- Iterate solution.
- Review results.
What is PAM and what are the steps involved?
- Establish computer representation
- Calibrate model
- Locate representative boundary conditions
- Undertake integrated simulations
- Express multi-variate performance
- Identify problem areas
- Postulate remedies
- Establish a reference model for each postulate
- Iterate from step 4
What are the steps involved in the calibration of the model?
- Input-output pairs for multiple simulation cases are recorded along with corresponding measurements of the outputs and time-matched weather data.
- This data is then used to create a meta-model that emulates the simulation tool being used. (A meta model is a model of a model and is used as it is a simplified model of the actual model).
- The meta-model is used to determine the input parameters that will cause the tool to reproduce the measured performance.
- These best-fit input parameter values are then applied to the initial model to get the calibrated model.
What is an IPV and what is included in it?
It is a tool that can bring together different aspects of the performance at any time. Some issues that can be viewed are: seasonal fuel usage, environmental emissions, comfort, daylight utilisation, risk of condensation and renewable energy contribution. This can show many parameters at once and therefore some of the negative effects can be seen to be related to other parameters.
What changes need to be made to design practice?
- Management of the application process (who does what, where and when.)
- The implementation of the PAM. This separates each step in a controlled way.
- A formal method to translate simulation outcomes to design modification. (ie from the simulation outcomes, we should have a method of using the results to change the design.)
