Overview · Fundamentals · Documentation · Examples

Solar collectors

A solar thermal system consists of the collector array, the store as the heat sink and the control. Unlike photovoltaics, the heat cannot be shifted freely: whatever is not used or stored promptly is lost. The benefit is therefore determined not by the installed area but by the interplay of area, storage and demand profile.

Collector types

Flat plate collectors are the standard for hot water and heating support in residential buildings: robust, inexpensive and efficient at moderate temperature differences. Evacuated tube collectors have lower heat losses and therefore keep their advantage at high operating temperatures or low ambient temperatures – for process heat or solar heating support in winter – but are more expensive. Which type performs better depends on the required temperature level and can be compared directly using the stored characteristics.

Orientation and tilt

Ideally the array faces south; deviations of up to about 30 degrees cost only a few per cent of yield. The tilt determines the seasonal distribution: a shallow arrangement favours summer yield and thus hot water preparation, a steeper arrangement of about 45 to 60 degrees shifts the yield into the transitional season and supports space heating. EPOS-Plan calculates the angle of incidence on the tilted surface for every hour from the solar position and the orientation, so variants can be compared directly.

Efficiency model

Yield calculation follows the collector efficiency according to EN 12975. From the optical efficiency – the share of irradiance that enters the collector at all – the temperature-dependent losses are subtracted, growing linearly and quadratically with the difference between collector and ambient temperature. At low irradiance and large temperature difference the efficiency becomes zero: the collector then delivers nothing. The model is complemented by the incidence angle modifier, which accounts for the shallow angles in the morning and evening hours, and by pipe losses between array and store.

The relevant characteristics – optical efficiency, the two loss coefficients and the incidence angle modifier value – come from the manufacturers’ test reports and are stored in the collector catalogue. Own data sets can be created and manufacturer data imported according to VDI 3805.

Storage and use

The store decouples yield and demand. Small volumes are sufficient for hot water support; heating support requires larger stores and clean stratification. In combination with a CHP unit or a heat pump the generators compete for the same store – solar thermal then reduces the runtimes of the base-load generator, which affects the economics of a CHP unit. This conflict can be quantified in the simulation by varying the order of the generators and the storage thresholds.

Modelling in EPOS-Plan

A collector array is described by collector type, number of modules, aperture and module area as well as tilt and azimuth; several arrays with different orientations are possible. Besides the usable heat production and the coverage share, the simulation reports the surplus – the heat that cannot be used in the respective time step. This figure is the real yardstick for sizing: if it rises markedly, the array is too large for the given demand and storage profile.