In this chapter, we briefly consider specific conditions typical for operation of ventilation systems in countries with cold climate.
4.3.1 Heat Recovery
According to the previous studies [14,17,20,21], the heat losses through venti- lation system of buildings increase at larger temperature differences between indoor and outdoor air. In cold climates, this difference can often exceed 50 °C. Indeed, Fig.4.3 clearly demonstrates that in many world’s territories the average winter temperature is as low as (−30) to (−45)oC. It is reasonable to assume that these losses correlate with the number of heating degree days HDDẳHðTbasToutị, whereHis the heating period,Tbasis a base indoor temperature (in Russia it isfixed at 20 °C [24]), andŤoutis the outdoor temperature averaged over the heating period (see Table4.1). In Russia, the heating periodH= H8 isfixed as a cold season with the mean daily outdoor temperature Tout ≤8 °C. For instance, in Moscow, H8 = 214 days and HDD (Tout≤ 8 °C) = HDD8 = 4943 (°Cday)/year, in Novosibirsk—230 days and 6601 (°Cday)/year, and the average HDD8 in Russia is 5140 (°Cday)/year (Table 4.1).
In other cold countries, the heating periodHis calculated in a somewhat different way and the base indoor temperature isfixed lower (16–18 °C) than in Russia. As a result, the so-called HDD10 (an arctic heating degree days below 10 °C) differs from HDD8 as displayed in Table4.1for the selected Russian cities. For comparison, the same indicator (in (°Cday)/year) for several European capitals is as follows: London
—1860 [25], Helsinki—2601, Oslo—2324, Stockholm—1847, Copenhagen—1311 [14]. Thus, on the largest part of the territory of Russia, the demand for thermal
Table 4.1 Characteristics of heating season in selected Russian cities (the data were taken from [14,26]) and some useful estimations (see text below)
City H8
(day/year)
Ťout(°C) HDD8, [(°Cday)/
year]
HDD10 [(°Cday)/
yr]
Tmina (°C) Wav
(kW)
Q(GJ/year)
Moscow 214 −3.1 4943 2964 −28 1.2 22.2
St. Petersburg 220 −1.8 4796 2750 −26 1.1 21.6
Rostov on Don
171 −0.6 3523 1676 −22 1.0 14.7
Yekaterinburg 230 −6.0 5980 3759 −35 1.3 26.9
Novosibirsk 230 −8.7 6601 4115 −39 1.5 29.7
Yakutsk 256 −20.6 10394 – −55 2.1 46.8
Khabarovsk 211 −9.3 6182 4425 −31 1.5 27.8
aThe reference outdoor temperature used for designing the regional heating systems
energy needed for space heating is higher than in the Northern Europe. In much the same way, very cold territories are located in the northern parts of Canada and USA (Fig.4.3), e.g., for Barrow, Alaska HDD10 = 11,055 (°Cday)/year that is similar to Yakutsk, Russia (Table4.1). One may expect that the most efficient measures to reduce the ventilation heat losses have to be made in the territories with large HDD values.
The data of Table4.1allow a brief estimation of the powerWavnecessary for heating outdoor air up to the base indoor temperature of 20 °C, averaged over the Fig. 4.3 Average air temperature in January in the World [22] (a) and territory of the ex-USSR [23] (b)
heating period H8. For a reference apartment/house of 100 m2 total area (or the total volume V= 300 m3) with the air exchange E of 0.5 h−1, this power is WavẳCpVE TbasTout
, where the air-specific heat Cpis assumed constant and equal to 1.25 kJ/(m3K). This power is displayed in Table4.1 together with the annual amount of heatQnecessary for the outdoor air preheating. The annual heat demand only for preheating of outdoor air is larger that the maximum space heating standards established for a low-energy building. For instance, according to the German rules, the latter is 50 kWh/(m2year) that corresponds to 18 GJ/year for the reference 100 m2 house. These estimations clearly demonstrate that recovery of ventilation heat is strictly necessary in cold countries in order to satisfy the stan- dards of energy efficient building and make bills for heating acceptable.
4.3.2 Moisture Recovery
Another problem that appears during cold winter is continuous loosing of moisture through ventilation system [21, 27]. Indeed, the absolute humidity of supplied (outdoor) air dout is extremely low (e.g. ≤0.29 g/m3 at −30 °C) that results in dramatic reducing the indoor relative humidity (down to 10–20%, Fig.4.4) in winter season that is far out of the borders of indoor thermal comfort. The indoor RH is also sensitive to the presence of central heating period (Fig.4.4).
Let us estimate the massMof water vapour that has to be generated inside the mentioned reference house in order to maintain an acceptable RH of 40% (at Tin= 20 °C, this corresponds to the indoor absolute humiditydin= 7.1 g/m3) if the outdoor tempera ture is −30 °C: M= VE(din−dout) = 1.02 kg/h. A power of 0.7 kW is required for the forced vapour generation (evaporation) that is ca. a half of the average power spent for air preheating (Table4.1).
Fig. 4.4 Typical evolution of the indoor relative humidity in a room with natural air infiltration (Novosibirsk, Russia: 55° 02′N, 83° 00′E).
The RH data between September 4, 2007, and October 25, 2007, were not recorded
Therefore, in cold countries, it is extremely important to organize not only efficient heat exchange between exhaust and supply air flows, but also a partial exchange of moisture between theseflows to maintain the indoor humidity within the comfortable range.
The partial transfer of water vapour from the exhaustflow to the supply one is also necessary in order to avoid a frost formation near the outlet of exhaust airflow of ventilation system. It may happen when outdoor air is cooled below a water freezing temperature [28,29]. Table4.2shows that a dew point of the exhaust air is rather high, and to avoid or at least alleviate a frost formation the absolute humidity of the exhaust air has to be reduced down to 0.2–0.5 g/m3.
4.3.3 Modern Heat and Moisture Recovery Approaches
As discussed above, for winter season in severe climates (typical for Russia, the Northern Europe, the USA, and Canada), the difference ΔT between indoor and outdoor temperature can reach 50–60 °C or even more that leads to enormous heat losses and freezing of moisture at the system exit. As a result, common heat recovery units integrated in ventilation systems may not be capable to work at these conditions. Moreover, such systems are not able to manage the indoor humidity, which dramatically reduces in winter season that greatly disbalances the indoor heat comfort. Thus, tofill these three main gaps in the current ventilation techniques, the following actions should be performed:
– efficient exchange of heat between the exhaust and supply airfluxes to reduce heat losses,
– reasonable drying of the exhaust air to avoid ice formation at the system exit, – moisturizing the supply air to provide indoor conditions of human thermal
comfort.
There are many technologies suggested and studied for resolving these prob- lems. Here we just briefly survey the current state of the art in this field. The interested reader is referred to appropriate reviews, e.g., Ref. [21] which specifically addresses energy recovery systems in apartment buildings located in cold climate countries using central air handling units. In that review, heat exchangers recov- ering sensible heat are compared with energy exchangers with recovery of both sensible and latent heat, whereas here we consider only the energy exchangers.
Table 4.2 Dew point of the exhaust air at various RH values andTin= 20 °C
RH (%) 1 2 5 10 20 30 40 60
din(g/m3) 0.18 0.36 0.88 1.8 3.6 5.4 7.2 10.8
Dew point (°C) −34.5 −28 −19 −11 −3 +2 +6 +11
4.3.3.1 Energy wheels
Energy wheels are regenerative exchangers aimed to exchange heat and moisture between supply and exhaust air streams in ventilation systems of buildings (see the fundamentals in Refs. [30,31]). When humid and warm exhaust airflows through the adsorbent coated wheel in the return section, both moisture and heat taken from this stream are adsorbed and stored in the wheel. Therefore, the frost formation in this section becomes less likely [30]. As the wheel turns, the part, where the sensible and latent energies have been accumulated, appears in the supply section where cold and dry outdoor air takes the stored heat and moisture and becomes more warm and humid. The wheel effectiveness is typically 50–85% for both latent and sensible heat recovery [32] that is sufficient for the majority of applications.
These units exhibit a low pressure loss only of 50–200 Pa that is lower than for most of other air-to-air heat exchangers [30].
Some shortcomings are inherent for energy wheels, namely, moving parts that need appropriate maintenance, carryover leakage due to penetration of a small fraction of exhaust air to inlet air that reduces the efficiency and can be a source of odour transfer [33].
4.3.3.2 Membrane energy exchangers
Another solution to resolving the mentioned severe problems is the application of porous polymer membranes partially permeable for moisture. Membrane energy exchangers are a new class offlat plate energy exchangers proposed by Zhang [34], which already has passed away from fundamentals to engineering applications [27, 35,36].
Such energy exchangers are similar to traditionalflat heat exchangers with the difference that the former use polymeric plates permeable for water instead of common metal sheets. Partial transfer of water through the membrane helps in drying the exhaustflux and moisturizing the supply one. The transfer is driven by the gradient of moisture concentration in the streams and depends on the compo- sition, shape, and size of the membrane.
These energy exchangers are simple and reliable. There are no moving parts, and no external heat for regeneration of desiccant is required. However, the problem of odour can still be important because there is a mass transfer through the membrane.
Frosting in the membrane, its mechanism and limits also need more theoretical and experimental investigations, because so far the majority of tests have been per- formed in warm and humid climates, like in Hong Kong [37], rather than in cold and dry ones. At least, paper membranes are expected to be not appropriate for cold climates due to the ice formation on the membranes [21].
Despite significant progress achieved for the past decades in developing energy exchangers for cold climates, some of which have already appeared in the marked, still there is much room for further improving the existing devices and development of new advanced energy exchangers.