System Concept With Storage Tank

If the simulation results of the reference cases are plotted as a function of the utilization ratio, all simulated points almost fall onto a single line that is decreasing with increasing utilization ratio. Towards the left, the collector area is increased, to the right it is decreased. The larger the collector area and the smaller the heat demand is, the higher is the solar fraction. High levels of utilization signify low degrees of solar fraction and vice versa. The specific yield shows the well-known behavior running contrary to the solar fraction.

Figure 5. Nomogram to determine the collector area and the solar fraction and at the same time the specific
collector yield. The graph is based on a specific storage tank volume of 50 liters per m2 of collector area.

The only points that are slightly lower than the overall trend curve are the ones that represent Case 4 (with high internal loads). In this case, the internal loads occur always during the daytime (i. e. at

the same time when there is solar yield). Because of the internal loads there is no heat demand during working hours but only at night and on the weekends. Therefore, the solar energy can never be used directly in the building but always has to be stored for later use. Therefore, more storage tank losses occur compared to a building that has space heating demand also during the day.

The light-colored areas in the diagram show the typical range of solar fractions and specific collector yields that are reached with systems using the storage tank concept. Particularly in industrial buildings, economic considerations dominate that is why systems should be designed in accordance with the optimum ratio of cost-to-benefit (orange area, between approx. 15 and 40% solar fraction). Degrees of solar fraction of less than 15% are outside the cost-to-benefit optimum since the (slight) rise in the specific yield does not make up for the higher specific system costs of a smaller solar thermal system and would thus lead to higher solar heating costs.

Handling of the Nomogram: The nomograms can be used in two different ways:

Determination of solar fraction: If the annual heating requirement has been determined, the utilization ratio for a particular planning project can be calculated by dividing this value by the collector area. If a vertical line is drawn through the point of the determined utilization ratio, then the intersection with the curve of the solar fraction is obtained and the value can be read off on the left ordinate. The same is true for reading off the specific yield on the right ordinate.

Determination of collector area: If a desired solar fraction constitutes the starting point then a horizontal line can be placed at the corresponding height. The point intersecting with the curve of the solar fraction then allows the necessary utilization ratio to be read off on the abscissa. The necessary collector area is obtained by dividing the annual heating requirement for space heating by the utilization ratio. The solar storage tank volume of 50 l/m2 is directly proportional to the collector area.

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