What is VELA and how does it fit into your development process?

VELA is a web-based tool that predicts the heat loss of thermal energy storage tanks. The vision behind is to assist you during the design process for new products. Instead of having to build and test multiple prototypes to evaluate design options, VELA helps during the early stages of development to quickly numerically evaluate the options and make informed decisions to chose, improve or reject options.

VELA uses a simple, accurate, yet flexible heat transfer model to calculate the heat losses according to the applicable standards for your individual tank. The simplicity and intuitive usage of the tool allow you to generate meaningful data without the need of deep knowledge in the areas of heat transfer and simulations. The reports generated during the simulation show how different parameters affect the performance of your product and guide you towards optimizing it.

If you need further help, please contact the UltrasimWeb Team.

How the model works
VELA utilizes a one-dimensional heat transfer model. This means that the tank is discretized into a number of equally sized cylinder elements. In every timestep the energy balance is solved for each element. The figure below shows the discretized representation of an exemplified tank as well as the contributions to the energy balance of each cell. During each of the solved timesteps every cell interacts with its adjacent cells as well as its surrounding. Additionally heat exchangers, patches and connections can be accounted for. One-dimensional modeling has advantages, as changes in operating conditions or tank geometry are processed within a few seconds and results can be visualized immediately. After a simulation several parameters like the cell temperatures can be visualized (see section VELA Outputs).

Energy Classification
To obtain a virtual energy classification in accordance with DELEGIERTE VERORDNUNG (EU) Nr. 812/2013, VELA implements the currently technically applied test procedures.

The figure above shows an example energy label for a thermal energy storage tank. The classification spans from A+ to F.

VELA Inputs
The model inputs as shown below are structured into different modules: Tank, Filling, Insulation, Heat Exchangers, Connections, Patches and Simulation. Each module contains several parameters that can be set according to your tank in consideration. The chosen defaults allow you to test VELA without having to use your own parameters. Please find further information about model parameters in the following sections.

Tank

The main geometrical characteristics of the tank, such as height, diameter, wall thickness, are defined in the tank module. By selecting a material from the available set the thermal properties of the tank insulation are set.

Filling

In the filling module, you can select a fluid, the tank is filled with. The default which is likely to work in most cases is water.

Insulation

Since the polyurethane foam insulation has the greatest influence on the result, precise material property measurements are of great importance. Thickness and insulation material must be selected. The Ultrasim material data base provides a large selection of state-of-the-art insulation materials that were carefully characterized.

Heat Exchangers

To comply with the common test standards, the heat exchanger module allows to specify a few heat exchanger properties like its location as well as its power output.

Connections

Connections are typically a heat leak. In VELA, connections are modeled as flange with or without insulating cover.

Besides the geometrical properties, the materials for the pipe and the insulation can be selected in the connection dialogue.

Patches

Patches are a versatile feature. They can be used to model inserts that displace polyurethane foam insulation.

The tank surface area covered with a patch (displayed as hatched doughnut in the image above) is considered insulated by the patch. Thus, the thermal conductivity of the patch material and its thickness define the heat loss as opposed to the properties of the main polyurethane insulation.

Patches can also be used to model thermal bridges such as feet.

In this case the foot consists of a beam with rectangular cross-section (area A) bent twice and welded to the tank. This foot can be modeled by creating two patches, with lengths t_1 and t_2 respectively and cross-sectional area A_1 = A_2 = A.

Surrounding

Ambient temperature as well as the material properties of the surrounding can be provided here. The ambient temperature has a critical influence on the overall heat losses since its difference to the inner temperature is the main driving force of heat transfer. By choosing one of the standard test norms, the values are set accordingly.

Simulation

Test norms as well as simulation time can be specified here. Typically, the defaults can be used.

Vela Output - Results Dashboard

A wide variety of parameters can be accessed in the results dashbord.

Summary

The most important metric is the following chart combined with the energy label of your specific tank. On the left in the section Energy Classification you can see an area chart showing the allowed limits for heat losses in all energy classes as a function of the tank inner volume. The simulation result of your tank is visualized by a black dot. With this representation it becomes possible to how much better the tank needs to be insulated to reach a better energy label.

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