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Flat solar collectors, usually including heat accumulators, are the most common thermal solar modules. However, extensive research is still underway to improve their technical and economic characteristics. The complexity of solving this problem is primarily due to the relatively low energy density of solar radiation as an energy source, which also leads to significant thermal engineering complexity of low-temperature solar installations, including solar collectors. In particular, this requires the improvement of both computational methods and models for determining the characteristics of solar collectors and heat accumulators, both “exact” and approximate (Moss, Henshall, Arya, Shire, Eames & Hyde, 2018; Basak, Mitra, Ghosh, Sarkar & Neogi, 2016).
So, in approximate calculations of the characteristics of solar collectors, a simplified formula for the instantaneous efficiency of a solar collector in a stationary mode of its operation is widely used:
(1) where
η0 is the optical efficiency;
U is the coefficient characterizing the heat losses of the solar collector surface and has the dimension of the convective heat transfer coefficient;
ΔT is a certain temperature difference and
ES is the density of solar radiation incident on the solar collector. In some cases, the nonlinear term of
ΔT is also included in (1). The
U coefficient, also called a measure of the thermal technical perfection of a solar collector, depends on the parameters of the solar collector and the external environment and characterizes the heat losses of the solar collector by convection and radiation and is currently determined for a specific collector only experimentally.
As is known, energy (heat) accumulators are needed to improve the efficiency of solar installations, including solar collectors (water heaters, air heaters, dryers). Work on the creation of heat accumulators has been carried out practically since the development of solar thermal installations. The main types of short-term, medium-term and seasonal heat accumulators have been developed.
Currently, the tasks of creating effective designs of heat accumulators are being solved, as an element that is optimally combined with solar thermal installations and their purpose, including heat pumps, technical and economic assessments of the use of such systems for heating and air conditioning are underway (Guo, Yang, Xub, Torrens, & Hensen, 2017; Ma, Bao & Roskilly, 2018; Schramm & Adam, 2014; Stutz, Pierres, Kuznik, et al., 2017; Malan, Dobson & Dinter, 2015; Herrando, Ramos, Freeman, Zabalza & Markides, 2018; Ojike & Okonkwo, 2019; Wajahat & Ali, 2019). In this case, one of the tasks of designing heat accumulators is to ensure its minimum heat loss.