Influence of Energy Load on Performance of a Solar Water

INFLUENCE OF ENERGY LOAD ON PERFORMANCE OF A SOLAR WATER

HEATING SYSTEM

P. Víg and I. Farkas

Department of Physics and Process Control

SzentIstvánUniversity, Páter K. u. 1., Gödöllő, H-2103 Hungary

Tel.: +36 28 522055, E-mail:

In middle of Europe the meteorological conditions are favourable for the use solar energy by flat-plate solar collectors. Applying such systems it is important issue that how much energy can be gathered by the system. In the present work the influence of the energy load associated with hot water consumption is studied comparing to the maximal daily produced energy by solar collector.

The data used during the tests were measured July-November in 2006 and April-May in 2007. The studied solar system was installed at the Department of Physics and Process Control, SzentIstvánUniversity, Gödöllő, Hungary. This set-up serves as a domestic hot water system, the area of flat-plate solar collector is 1.65 m2 with south orientation and 45o inclination angle. The storage tank has no electrical heater, the volume is 150 l. For the more efficient realisation of the stratification a special internal heat exchanger was installed inside the tank.

The collected energy by solar collector was calculated from difference of collector outlet and inlet temperatures and mass and specific heat of solar liquid. The consumed energy by hot water load was calculated from the difference of load and tap water temperatures and mass and specific heat of water. During plotting the daily specific energy load versus produced energy data pairs a boundary curve can be observed, and beyond of that there are no additional points.

In order to derive equation of boundary curve the data of non-optimal condition days were filtered. The following filters were used: day data when the solar loop did not work; no-load day data; data when the solar radiation was not durative high; high initial stored water temperature with small load and high load but too late. The produced daily maximal specific energy by the collector can be determined from the given daily energy load data with the help of equation of curve which was fitted on the residual points.

It can be observed that the daily specific energy income of collectors can be enlarged only up to a given limit by increasing of the load. Beyond this limit the energy income can not be increased even under ideal radiation, initial storage temperature and optimal load, as well. The relationship shows saturation character from which the maximum daily specific energy income and the ideal daily specific energy load can also be evaluated. The latter value for studied flat-plate collector was determined as 2,1 kWh/m2.

In case of arbitrary solar thermal system the introduced method is suitable to determine the equation of saturation curve, the maximal daily produced specific energy and the optimally demanding energy load.