4.4 The New Approach to Regeneration of Heat
4.4.3 Designing and Testing of VENTIREG Prototypes
To study and optimize the heat and moisture recovery, four experimental units with the air flux up to 40 (I), 135 (II, III) and 220 (IV) m3/h were built and tested (Fig.4.8). The adsorbers werefilled with composite IK-011-1 selected as the best water buffer. The heat storage part was organized as a bed of loose particles of gravel of irregular shape (4–7 mm in size). Units I–III could operate only under intermittent mode because they had only one adsorber. Unit IV was designed for continuous operation and had two“adsorbent—HSM”beds working in opposite modes (Fig.4.5).
Typical evolution of the air temperatureTduring cycling operation is displayed in Fig.4.9(unit I). The steady-state regime is set after 10–20 min. All the regular tests were performed under this regime. The half cycle timeΔτ increases at larger ΔT0and longer contact time (or smallerRe-numbers) as shown in Fig.4.10plotted for unit III.
Figure4.11 displays typical evolution of the air temperature and absolute humidity as function of timetfor cycling (steady state) VENTIREG operation (unit III). During the outflow mode, the indoor air at T = 20 °C and d= 5 g/m3 was passed through the adsorbent and HSM. At the beginning of this mode, the air was dried and cooled down to 0.5 g/m3and−26 °C, respectively. During thisfirst half of the cycle, the air temperature and humidity were gradually increasing until the temperature reached−18 °C (atΔτ ≈580 s) that was 10 °C higher than the out- door temperature −28 °C (ΔTo= 10 °C). Then, the system was switched to the inflow mode: the outdoor air atT= −28 °C andd= 0.35 g/m3was passed through the HSM and adsorbent (see Fig.4.5). At short times, the air was humidified and heated up to 4.8 g/m3 and 20 °C. Then, its temperature and humidity were Fig. 4.8 View of VENTIREG units I–IV: the inlet airflow rate up to 40 m3/h (a), 135 m3/h (b, c), and 220 m3/h (d)
gradually decreasing until the temperature reached 10 °C that was 10 °C lower than the room temperature, and the next cycle was started (Fig.4.11).
The air fluxes during the inflow and outflow modes were equal, and the data given in Fig.4.11 allow estimation of the efficiencies of heat regeneration θ=SACDE/SABDEand moisture regenerationβ=SKMOP/SKLOP, whereSis the area of appropriatefigures. As shown in Fig. 4.11, theθvalue is rather large: almost all heat is accumulated during the outflow mode and returned to the room during the inflow mode (only the amount of heat corresponding to area ABC is lost, Fig.4.11).
The heat regeneration degree would be even larger atΔT< 10 °C.
Unit III was continuously tested under climatic conditions of the Western Siberia (Novosibirsk city: 55° 02′N, 83 °00′E) during a full HDD season of 2005–2006. As the outdoor temperatureTout and, hence, the difference ΔTMAX=Tin− Tout were not constant during thesefield tests, it is convenient to present the heat regeneration degree θ as a function of the dimensionless temperature difference DT~ ẳ DTo=DTMAX (Fig. 4.12a). The collected data demonstrated that a high heat regeneration efficiency (θ> 0.9) can be readily obtained atDT~ ≈ 0÷0.25 that is a very common case. The degree of moisture regeneration was lower (Fig.4.12b).
0 10 20 30 40 t, min
-8 -4 0 4 8 12 16 20
T,
ΔT0
ΔT0
Δτ - 1
- 2 - 3 Fig. 4.9 Typical evolution of °C
the air temperatureTduring cycling operation of unit I (1 indoor side,2between the adsorbent and HSM,3 outdoor side).Tin= 20.5 °C, Tout=−8 °C,ΔT0= 3 °C
10 100 1000
Re 100
1000 10000
Δτ, s
− ΔT = 5.0
− ΔT = 7.5
− ΔT = 10.0
− ΔT = 12.5
− ΔT = 15.0
°C
°C
°C
°C
°C
Fig. 4.10 Half cycle timeΔτ (s) as a function of theRe- number (unit III) [20]
For generalization, it is similarly presented in Fig.4.12b as a function of dimen- sionless difference in the absolute humidity Dd~ =Δd/ΔdMAX, where Δd is the difference in the absolute humidity at the moment offlux switching,ΔdMAXis the maximal difference in the absolute humidity during a particular experiment.
Nevertheless, theβvalues larger than 0.7 were recorded atD~d = 0÷0.7, so that an efficient regeneration of both heat and moisture took place in VENTIREG unit III. It is important that both these efficiencies can be easily and purposefully varied by managing the half cycle time Δτ which depends on ΔTo and the Re-number (Fig.4.10), and the range of Δτ-variation can be rather wide (3–70 min).
It is somewhat surprising that for unit III the values of both θ and β can be approximated by a unique equation
hðbị ẳ10:38DTeðD~dị 0:12DTeðD~dị2: ð4:2ị The fact that bothθ and β are described by the same equation may indicate a strong coupling of heat and mass transfer processes when a humid air passes through an adsorbent layer [43]. At the same ΔTo, the efficiency of heat regener- ation increases with the rise ofΔTMAX that means the VENTIREG unit operates better at colder climates. Indeed, ifΔTo= 10 °C, atΔTMAX= 50 °C,DT~ = 0.2 and θ≈0.92, atΔTMAX= 30 °Cθ≈ 0.87, whereas atΔTMAX= 20 °C theθvalue is again lower (0.79), yet very promising for practice.
Fig. 4.11 Evolution of the air temperatureTand the absolute humidityd as functions of time tduring cycling operation of VENTIREG unit III (1indoor side,2at the middle of the unit,3 outdoor side).Tin= 20 °C,Tout=−28 °C,ΔT0= 10 °C. Airflux 123 m3/h
It is interesting to note that equations similar to Eq. (4.2) were obtained during field tests of VENTIREG units II and IV. This is probably because of the fact that heat and mass transfer processes and the main geometrical proportions [44] of the all three units are similar.
Very importantfinding of thefield tests of VENTIREG units II-IV is that no ice formation inside the units or at the units exit was revealed during the whole winter period. Anotherfinding is a very little maintenance required for the VENTIREG units tested.