Using the heat pumps as a renewable source of energy

SIMONA DUICU

Department of Manufacturing Engineering

Transylvania University of Brasov

500036 B-dul Eroilor, nr.29, Brasov

ROMANIA

Abstract: In the paper are shown some advantages of using heat pumps as a renewable source of

energy. Also it is

make a short presentation of cooling and heating systems together with the main

sources of thermal energy.

Keywords: heat pump, renewable energy, and thermal energy.

1 Principle of cooling systems

Cooling systems and heat pumps are thermal

machines, which has the role to take warmth

from environment with a lower temperature

and to yield it to other environment with

upper temperature as it shown in Fig.1. This

can be considerate the simplest model cooling

system.

Fig.1 Principle of cooling system.

Because warm and cold source has an infinite

thermal capacity, their temperatures remain

constant even they are changing heat. The

warm flow, which is absorbed from the cold

source, is Q0 and the warmth flow

yielded to warm source is Qk.

According with the second low of

Thermodynamics, named Increase of Entropy,

which it states that heat cannot of itself pass

from a colder to a warmer body, for

warmth transportation is necessary a energy

consumption P.

To can take warmth from the cold source, the

cooling medium must have a lower

temperature than it.

During this process, the cooling medium can

have two types of behavior:

- increase its temperature (Fig.2a);

- maintaining its temperature (Fig.2b).

where:

S is warmth changing surface, tr is cold source

temperature.

To can maintain unchanging the coolant

temperature, during the process of taking

heat is necessary to transform its state, called

vaporization.

The absorbed heat Q0 in both situations is

given by [2]:

Q0 = m1·cp·Δt [kJ] (see Fig.2) (1)

Q0 = m2·r, [kJ] (see Fig.3) (2)

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a) b)

Fig.2 Types of cooling medium behavior.

where:

m1 = quantity of coolant which is heated [kg],

Cp =

specific warmth [kJ.kg-1.K],

∆t = coolant temperature variation between two stages [°C],

m2 = quantity of coolant which vaporized [kg],

r = latent vaporization heat of coolant at temperature t0.

To make an efficient thermal transfer, ∆t is limited at a few degrees. To can yield heat to

warm

source the coolant must have a higher temperature that it. During the process of transfer the

heat to

the warm source the coolant can have two states of behavior:

decrease its temperature (Fig.3a);

maintaining constant its temperature (Fig.3b).

a) b)

Fig.3 States of behavior of coolant during transfer to warm.

The yield heat Qk is given by [2]:

Qk = m1·cp ·∆t, [kJ], (Fig.3a) (3)

Qk = m2·r [kJ], (Fig.3b) (4)

where:

m2 = quantity of coolant which condenses.

To maintain constant coolant temperature

during the yield heat process is necessary

changing its state, called condensation.

2 The main elements of a cooling

system

The condensation pressure is higher

than that

of vaporization. For this reason in this

system is consuming energy to increase

vapor

pressures from vaporizer taking heat

from cold

source till the pressure from

condenser where

will give heat to warm

source. This process is

achieved in

compressor.

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Fig.4 Elements of cooling system.

After compression, the coolant vapors give

the heat in condenser to cold source and

condense when it becomes liquid. From

energetic viewing, coolant relaxation is

made by detentor, that has the advantage of

producing mechanical energy and power,

which compensates some quantity, needed to

compressor. Cooling systems and heat

pumps have four main elements: vaporizer

(V), compressor (C), condenser (K), and

detentor (D), (Fig.4).

3 Comparison between cooling

systems and heat pumps

In principle the inverted cycle of function for

these two systems is identical. The difference

is only temperature heat sources level from

that of environment, ta [°C], respectively Ta

[K]. In Fig.5 are shown three schemes of

installations, which are working with

inverted thermodynamic cycles.

a) Cooling installations with cold source

temperature tr [°C] or Tr [K], equal with

vaporization temperature t0 [°C] or T0[K],

lower than environment temperature ta [°C] or

Ta [K].

b) Heat pump with warm source

temperature tc [°C] or Tc [K], equal with

condensation temperature tk [°C] or Tk[K],

higher than environment temperature ta [°C] or

Ta [K].

c) Combined systems, with cold source

temperature equal with vaporization

temperature, lower than environment

temperature.

Fig.5 Schemes of installations.

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4 Energy sources of heat pumps

Modern electric heat pumps obtain approx.

three quarters of the heat required for heating

from the environment, the remaining

quarter is drawn as electrical power for

driving the compressor

Fig.6 The effectiveness of the heat pump

The performance factor is a result of the ratio

between the transferred heating energy

(including the compressor heat generated by

the electrical power consumption) and the

energy used (power supplied to the

equipment), which describes the effectiveness

of the heat pump (Fig.6).

Irrespective of their type, heat pumps can

be viewed as equipment which raises the

temperature of a process medium from a low

to a higher temperature level using additional

energy,

There by making the heat content of the

medium useful. The main sources of energy

which are used by heat pumps are:

1. Water. The most efficient source for an

ecologic heating is underground water, if is

to a suitable depth. A constant temperature

of 80C- 120C makes water to be a thermal

energy for entire year and a small price. Its

efficiency is high and will be transported

from the well to heat pump and to drain

well situated at 15-m distance (Fig.7).

a) b)

Fig.7 The most efficient source for an ecologic heating.

2. Ground. The ground is a good energy

store, as underground temperatures ranging

from 7 to 13 °C are relatively even all the year

round. The thermal energy can be captured by

horizontal collectors or with a probes set

vertically into the ground at a depth of 15 m.

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(Fig.8). They are efficient even in the night

and in wintertime. Ground has the function of

heat capture. The pipes which deliver this

stored energy via a mixture of water and anti-

freeze (brine) to the evaporator of the so-called

brine/water heat pump are buried under

ground at a depth of between 1.2 m and

1.5 m. These ground collectors are made by

copper with thick wall.

a) b)

Fig.8 Capturing the energy from the ground..

3. Air. Outside air offers the least expensive

option for exploiting an energy source. Air is

supplied via a duct, it is cooled down in

the heat pump evaporator and then it is

returned to the ambience

(Fig.9). Modern air/water heat pumps can

provide heating energy down to an outside

temperature of minus 20 °C. However,

given optimum sizing at such low outside

air temperatures it can no longer meet the

central heating demand completely. On very

cold days, an electric heater inside the

calorific heats the heating water, which

was pre-heated by the heat pump, to the

selected flow temperature.

a) b)

Fig.9 Exploiting air as energy source.

Growing environmental awareness has focused

attention on the utilization of renewable

energies. As a consequence, heat pumps are

experiencing their own renaissance.

Today, heat pumps provide a reliable, cost-

effective and future-proof heating system,

which operates with particular environmental

responsibility.

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Fig.10 Growth rates for the German market.

In Switzerland today, every third new building

is equipped with an electrically operated heat

pump; in Sweden 7 out of 10 new buildings

rely on a heat pump. Growth rates for the

German market, too, are substantial, as shown

in Fig.12.

References:

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McGraw-Hill, new York, 1995.

[2] Vlisidis, A – A review of hybrid energy

systems for buildings. Doctoral thessis referate 1,

2006.

[3] Fragiadakis, G – Photovoltaic systems. Greek

edition, 2004.

[4] Patel, M. – Wind and solar power systems.

Florida, CRC Press, 1999.

[5] Mochammad Zakir, and ot. – Stand alone

system modeling. WREC 2002.

[6] Rodolfo, P. – Parametric analysis of wind

sitting efficiency. Wind engineering, 2003.

[7] Lun, I.Y.F – A study of Weinbull parameters

using long-term wind observations. Renewable

energy 20 (2000).

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DESIGN, CONSTRUCTION, MAINTENANCE

DOI: 10.37394/232022.2022.2.8

Simona Duicu

E-ISSN: 2732-9984

Volume 2, 2022