Renewable Energy Sources for the mCCHP-SE-RES Systems - Design for Micro-Combined Cooling, Heating and Power Systems - page 102

Because the time interval of the ST panel production does not coincide with the
use of the DHW, it follows as a direct consequence, need of thermal energy storage.
Thermal energy storage systems store available heat by different means in an
insulated repository for later use in residential applications, such as space heating or
cooling, and hot water production. Thermal storage systems are deployed to over-
come the mismatch between demand and supply of thermal energy and thus they are
important for the integration of renewable energy sources. Thermal storage can be
subdivided into different technologies: storage of sensible heat, storage of latent heat,
and thermochemical ad and absorption storage [ 12 ]. The storage of sensible heat is
one of the best known and most widespread technologies, with the domestic hot water
tank as an example. The storage mediummay be a liquid such as water or thermo-oil,
or a solid such as concrete or the ground. Thermal energy is stored solely through a
change of temperature of the storage medium. The capacity of a storage system is
de

c heat capacity and the mass of the medium used. Latent heat
storage is accomplished by using phase change materials (PCMs) as storage media.
There are organic (paraf

ned by the speci

n) and inorganic PCMs (salt hydrates) available for such
storage systems. Latent heat is the energy exchanged during a phase change such as
the melting of ice. It is also called

“ ”

heat, because there is no change of
temperature during energy transfer. The advantage of latent heat storage is its
capacity to store large amounts of energy in a small volume and with a minimal
temperature change, which allows efficient heat transfer.
Sorption (adsorption, absorption) storage systems work as thermochemical heat
pumps under vacuum conditions and have a more complex design. Heat from a high-
temperature source heats up an adsorbent (e.g., silica gel or zeolite), and vapor
(working

hidden

fluid, e.g., water) is desorbed from this adsorbent and condensed in a con-
denser at low temperatures. The heat of condensation is withdrawn from the system.
The dried adsorbent and the separated working

fluid can be stored as long as

desired. During the discharging process the working fluid takes up low-temperature

heat in an evaporator. Subsequently, the vapor of the working fluid adsorbs on the

adsorbent and heat of adsorption is released at high temperatures. Depending on the
adsorbent and working

fluids, the temperature level of the released heat can be up to

200 C and the energy density is up to three times higher than that of sensible heat

storage with water. However, sorption storage systems are more expensive due to
their complexity. For residential use the thermal energy storage with the domestic
hot/cold water tank is one of the best known and most widespread technologies.

°

3 Biomass

Ion V. Ion

Biomass, as solar energy chemically accumulated in vegetable or animal matter is
one of the most precious and diversi ed resources on the Earth. It provides not only

food but also energy, building materials, paper, medicines, and chemicals. The term