Page 50 - Low Temperature Energy Systems with Applications of Renewable Energy

38 Low-Temperature Energy Systems with Applications of Renewable Energy

main heat supply being done by means of the heat pump. When the ambient temperature was

lower than the set limit value, say, lower than 0 C, the supplemental heat generator began to
operate.
• Bivalent alternative mode
Heating units with a bivalent alternative operating mode are also equipped with a heat
pump and the second heat generator. However, unlike the bivalent parallel mode the heat
pump and the second heat generator never operate at the same time. Meeting the annual en-
ergy demand is done equally between the heat pump and a traditional heating boiler. If the

ambient temperature is higher, for instance, than 3 C, only the heat pump operates. When
the temperatures are much lower, heat supply is completely provided by the heating boiler.
• Circulating pump selection
Regardless of the mode of operation, circulating pumps need to be carefully specified to
avoid system losses and possible failure. To choose circulating pumps correctly the hydraulic
calculation of heating system pipe laying should be done. General pressure of a circulating
pump includes the sum of resistances of the heating system and heat pump condenser. When
an accumulating tank is used, the circulating pump should have the pressure capability to over-
come the total loop pressure that includes the resistance of the heat pump condenser, the resis-
tance of the tank, regulating armature, pipelines, etc. Proper circulating pumps should be
installed for the heating loops of the heating system (“warmfloor”, radiators, water supply, etc).

1.8 Accumulation and transport of low-temperature
energy

The problem of storing the energy that is received during peak production hours and
then used in hours of inadequate production (especially relevant for solar and wind
generation) for heating and cooling systems is solved using energy accumulators.
In Germany, there are home photovoltaic systems (PVs) equipped with power stor-
age units, storage tanks used in heating and air conditioning systems to store heat and
cold. Heat-storage accumulators are most often used in systems with periodically un-
even energy demand, for example, over a 24-h period. Examples of cold storage accu-
mulators include air conditioning systems in industrial shops, and milk and dairy
product cooling systems at dairies. The unevenness of thermal loads on the corre-
sponding cooling systems is determined by the change in external heat flows for
24 h; in the case of dairies by the frequency of milk arrivals over 24 h, and the neces-
sity for its cooling and recycling immediately and in the short term.
One could install a refrigeration unit that has a cold productivity being equal to the peak
loadonthecoolingsystem.But then,duringtheperiodsof lowload,anexcessiveamountof
thermal energy (heat or cold) will be produced and there will be excessive electricity con-
sumption.It isbetter that the cooling system includes acold-storageaccumulator as analter-
native variant. In this case, the refrigeration unit is selected with a cold yield slightly higher
than its average value (per 24 h), but less than the required peak value. Then, for periods of
time when the thermal load on the cooling system is less than the cold productivity of the
system, the excess of produced cold can be directed to “charge” a cold-storage accumulator.
In periods of time when thermal load is greater than the cold productivity of the cooling sys-
tem, one can use the cold accumulated in the accumulator to meet the demand. When there
comes again a period of small heat loads, the accumulator is “recharged”.Suchasimple, at