6.8.5. Solar Thermal Systems

The Einstein ST module helps you to configure a solar thermal plant (the solar field plus the solar heat storage tank) , taking into account some operational conditions such as the surface area suitable for installation, the heat demand up to 200°C and the weather conditions such as the solar irradiance and the ambient temperature. In order to make it easier, the Design Assistant may define for you all the basic parameters needed to design the plant, e.g. the solar fraction, the type of collectors and the minimum unitary energy yield.

The solar thermal module window

6.8.5.1. Solar Collector Type

In the first quadrant “Solar collector” (top-left) the technical features of the solar thermal collector chosen in the current alternative are displayed. Besides the name of the solar equipment some additional information such as the optical efficiency, the linear and quadratic heat loss coefficients and the longitudinal incident angle (along the tube) and the transversal incidence (perpendicular to the tube) angle modifiers are given.

6.8.5.2. Graphical Representation of Demand and Supply

In the second quadrant (top-right) the graph shows the cumulative yearly heat demand curve up to about 200°C. Once the solar thermal plant is designed, the yearly solar energy delivered is also showed up (in orange).

6.8.5.3. Configuration of Design Assistant

In the third quadrant “Configuration of design assistant” (bottom-left) 3 designing criteria are shown that can be modified by the user:

• Target solar fraction [%] (default value: 50.0%): it is defined as the ratio of the annual thermal energy delivered by the system to the annual heat demand up to 200°C. Theoretically it may range between 0 and 100%.
• Solar collector type [%] (default value: any): 5 options are foreseen – Flat-plat collector, Evacuated tubes collector, Concentrating collector, pre-selected collector and any. When the pre-selected collector option is chosen by the user then it is mandatory to enter a specific solar collector clicking on the button “choose solar collector”. In the other cases, the tool selects automatically the solar collector that performs the best energy results among those available in the database, under the category specified by the user (e.g. Flat plate).
• Minimum annual energy yield [kWh/kWth.a] (default value: 300): it allows the user to specify the minimum solar energy supplied per kWth installed below which the plant is considered not economically viable, in given operating conditions.

6.8.5.4. Size and Characteristic Parameters of the Solar Thermal System

In the forth quadrant “Lay-out of solar thermal system” (bottom-left) the configuration of the solar thermal plant (i.e. according the Einstein tool terminology: “heat generation equipment”) either is shown after running the design assistant or can be set manually by the user specifying the following parameters:

• Installed capacity [kW]: it defines the size of the solar thermal plant.
• Efficiency of heat storage and distribution [%] (default value: 0.9): it roughly accounts for the heat losses in the storage tank and in the distribution pipelines.
• Solar buffer storage volume [m3] (default value: 0.05 m3/m2 of solar collector aperture area ): it defines the solar heat storage capacity . If not specified by the user, the total storage is automatically calculated multiplying the default value by the aperture area of the solar field. Using a V/S factor of 0.05 m3/m2, implicitly it is assumed that the heat demand is continuous, i.e. no major breaks occur .

6.8.5.5. Relevant Parameters of Heat Demand and Solar System Performance

In the fifth quadrant (bottom-right below the graph) some auxiliary information are shown in order to help the user in his designing process. The key-parameters visualised here are:

• Gross surface area suitable for installation [m2]: it is derived from the questionnaire eliminating, e.g., the shading surfaces or the tilted surfaces oriented to the North. The figure shown says how big is the installing ground and/of roof area.
• Maximum possible solar thermal capacity [kW]: it is the estimated maximum power that can be supplied by a solar thermal field sized on the corresponding Gross surface area suitable for installation. It has to be lower than the Installed capacity [kW].
• Solar fraction (up to 200°C) [%]: it is a simulation result that can be easily compared to the target solar fraction in order to check whether the solar thermal plant performs the expectations of the user or not. If the latter, the user has to change manually one or more design criteria.
• Annual energy yield [kWh/kWth.a]: it is a simulation result. In order to make the plant economically viable, it has to be higher than the minimum annual energy yield. Otherwise the user has to change manually one or more design criteria.
• Average system efficiency [%]: it is a simulation result. It is defined as the ratio of the annual solar system energy output to the annual solar irradiance on a tilted surface. Expected values in standard operational conditions range between 35 and 60%.
• Average operating temperature (coll.) [°C]: it is a simulation result. Lower the average collector temperature, higher is the expected system efficiency. This parameter can be useful to choose the most appropriate collector type. Flat-plates, for example, show a good behaviour at low temperatures, while at medium temperature it is recommended to install either high efficiency or concentrating collectors.

6.8.5.6. How to design a solar thermal system ?

The ST calculation module offers to the user at least the following options:

1. To design a new system fully automatically clicking on the ‘run design assistant’ button (top-left) and… Einstein will work for him/her! Indeed the design assistant is able to analyse a set of given (or estimated whenever the user forgets to enter all the questionnaire parameters) conditions, and to configure a suitable solar thermal plant.
2. To run the design assistant and, afterwards, to adjust manually the configuration proposed automatically by Einstein.
3. To choose first a solar thermal collector clicking on the ‘choose solar collector’ button, and then to run the design assistant clicking on the ‘run design assistant’ button w/o modifying the Target solar fraction and/or the Minimum annual energy yield configuration parameters (see above).
4. To define fully manually both the solar thermal collector clicking on the ‘choose solar collector’ button and the solar plant size specifying the lay-out parameters, i.e. the Installed capacity, the Efficiency of heat storage and distribution and the Solar buffer storage volume.

Clicking on the ‘choose solar collector’ button the solar thermal collector database is shown in order to allow the user to select an equipment. Pay attention: a new solar collector cannot be added into the database as soon as it is shown . Whenever the user wants to enter new data he/she has first to accede the solar thermal database via the main menu bar. Once the solar collector is selected, it can be deleted by right clicking on it.

The “choose solar collector” option and the solar thermal database view

Finally, the user can accept the solar thermal plant configuration by clicking the ‘ok’ button (bottom-left).