BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI
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68 Bogdan Horbaniuc et al<br />
1. Introduction<br />
Thermal energy storage is a technique used to match the energy supply<br />
(such as wind energy, solar energy, etc.) and the consumer’s energy demand<br />
because the former exhibits important fluctuations or cut-offs (periodical or<br />
random).<br />
Thermal energy storage (TES) is of two types: sensible heat TES (heat is<br />
stored as specific heat of the sensible storage material (SSM) – water, rock, soil,<br />
aquifers, etc.), respectively latent heat TES (heat is stored as latent heat of<br />
fusion of a substance that undergoes a phase transition – molten salts, paraffins,<br />
alloys, etc.).<br />
The immediate benefits of TES (Dincer & Rosen, 2011) derive mainly<br />
from the increase of generation capacity by a better match of the supply and<br />
demand energy curves (because very seldom a peak in demand can be<br />
synchronized with primary energy supply), and from the possibility to shift<br />
energy consumption to low-cost periods.<br />
The most important advantages of TES are (Dincer & Rosen, 2001),<br />
(Dincer, 2002): reduced energy costs and consumption, flexibility in operation,<br />
reduced operation costs, increased efficiency of equipment utilization, and<br />
reduced pollutant emissions and thus a reduced ecological impact.<br />
The performance of a TES can be assessed by means of parameters such<br />
as: storage capacity [kWh/m 3 ], storage density [kWh/m 3 , or kWh/kg],<br />
charge/discharge rates [kW], storage efficiency determined by means of heat<br />
losses [kW], and SSM thermal cycling (the capacity of the SSM to preserve its<br />
thermophysical properties during long periods of operation involving a very<br />
large number of charge/discharge cycles). Thermal energy storage in soil has<br />
some additional advantages such as: favourable thermophysical properties of<br />
the soil, availability of the SSM, minimum costs for the storage system set up<br />
(mere drilling and tube positioning costs), and practically unlimited storage<br />
capacity (by increasing the number of boreholes). The main drawback derives<br />
from the TES set up: it cannot be insulated and therefore heat losses are high.<br />
A borehole thermal energy storage (BTES) is a set of vertical tubes<br />
inserted in the soil in order to transfer heat to and to extract heat from the SSM<br />
represented by the soil itself. Basically, such thermal storage systems are best<br />
suited for seasonal (long term) storage, i.e. summer/winter, but short term<br />
storage (on a daily basis) may also be considered.<br />
The charge/discharge rates of the BTES can be assessed by determining<br />
the dynamics of the temperature field within the SSM. Therefore, different<br />
approaches can be considered involving the analytical treatment of the transient<br />
heat conduction or the numerical approach using the finite difference method<br />
that avoids the almost insurmountable mathematical difficulties of the analytical<br />
method.