Preconditioning of supply air by an earthtoair heat exchanger EHX

Ventilation inlet air can be tempered if it passes through a buried pipe before entering the building. The thermal efficiency A EHX of such a system is the ratio between the temperature difference between air entering and exiting the system Toutlet - Tinlet and the temperature difference between the ground at that depth and the ambient TGROUND - TAMB:

eEHX = eHR = (TOUTLET - TAMB)/(TGROUND - TAMB) [1°.19]

Tout is the air temperature after having passed the EHX. It is equal to the air temperature at the inlet of the buildings ventilation system.

In winter, these systems gain heat from the heat stored in the ground, while in summer, the ground can absorb excess heat and cool the supply air. Applications are suitable for both small and large buildings with mechanical ventilation, adjacent to which preheating/precooling pipes can be buried. Special software for appropriate design is available on the market. Typical values of thermal efficiencies vary between 0.4 and 0.3.

A special benefit of EHX is to warm up the ambient fresh air beyond 0°C in order to prevent freezing out of humid air in the exhaust duct of the heat exchanger unit.

The advantage of ground 'preheat' recovery is 'free' heating and cooling from the ground. The disadvantages of ground 'preheat' recovery are:

• installation costs;

• extra fan capacity; and

• the requirement of a maintenance/replacement strategy.

These four heat recovery techniques, combined with mechanical ventilation, can provide about 5.5 W of heat (if latent heat is included up to about 7.8 W) per m2 heated at a temperature level of 20°C (with an assumed air change rate of 0.4 h-1). With a heated floor area of only 30 m2, this is already more than enough for DHW preparation and can even contribute significantly to space heating. This is evident because the average heating power needed to heat DHW heating amounts to about 80 W/person (based on a hot water consumption of 40 litres per person per day and a temperature rise from 10°C to 50°C).

Systems with only a heat exchanger commonly use a cross-flow or counter-flow plate heat exchanger with thermal efficiencies between 60 per cent (cross-flow) up to slightly more than 90 per cent (only for counter-flow). Cross-contamination between the inlet and exhaust air paths must be strictly avoided. Leakage would compromise with fresh air supply and, perhaps also, the thermal efficiency. In Germany, especially for central ventilation units, examination procedures have been defined that test the air tightness of both air tracts against each other.

As already mentioned, the more that the exhaust temperature approaches the ambient air temperature, the better the thermal efficiency. Therefore, the exhaust air tract runs the risk of condensing room air humidity, which can subsequently freeze up the air channel and block the removal of stale air. To prevent freezing, an electric defroster can be installed at the cold end of the air supply tract, warming up the ambient air at the intake to slightly above 0°C. This, of course, needs electrical energy. It is always a trade-off whether a system with moderate thermal efficiency and almost no risk of freezing is better than a system with high thermal efficiency, but also requiring occasional electrical defrosting. Herein lies the appeal of a ground heat exchanger to ensure that incoming air is always above 0°C. The combined efficiency eHR + EHX depends on the individual efficiencies eHR and eEHX as follows:

Here, the subscripts 'GROUND', 'AMB' and 'ROOM' designate the sub-surface soil temperature, as well as the outdoor and indoor air temperatures. The better eHR is, the lower the achievable improvement of efficiency by an EHX. Since an EHX also needs some fan power and causes additional investment, here, again, a careful investigation of the pros and cons is appropriate. As an example: with Tground ~ 10°C, Tamb ~ 0°C and TROOM ~ 20°C (typical values for winter conditions), efficiencies of eHR = 0.90 and eEHX = 0.50 result for the combination of both systems in eHR + eEHX ~ 0.90 + 0.50 (1.00-0.90) 0.50 = 0.925. This is a marginal improvement in thermal efficiency above eHR ~ 0.90 and shows that the EHX mainly serves to avoid freeze-ups of the heat exchanger.

The most sophisticated system combines an earth heat exchanger, a heat exchanger and a heat pump. This provides the best possible heat recovery from ambient sources and waste heat that can be achieved. However, such a system is also the most expensive and uses the most electricity. Detailed investigations with regard to primary energy and cost effectiveness are therefore advised before a decision is made for such a very advanced HR system.

Generally, advanced ventilation systems need (more expensive) electric energy with a high primary energy (PE) factor of 2.4, up to 3.0, in order to recover (cheaper) thermal energy with a PE factor of 1.1 (for oil and gas). This means that the net energy savings (NES) of such systems cannot really be characterized by looking at thermal efficiencies or COP (HP) only. 'Net' means consideration of auxiliary energy and refers to the primary energy level. Therefore, NES has to be calculated from thermal energy gains 0EH in terms of its inherent primary energy, which have to be diminished by the (PE weighted) needed auxiliary energy 0EE. In most cases, 0EE is electrical energy:

NES = PE (thermal) • AEH - PE (electrical) • AEe [10.21]

In a similar way, the net energy savings can be evaluated on the basis of the different energy costs for thermal and electrical energy. These cost-related figures even seem to be more appropriate for the common practice than mere energy-based considerations are. Instead of NES, net cost savings (NCS) are defined as:

NCS = Cthermal • ^EH - Celectrical • ^EE [10.22]

with typically Cthermal - €0.045/kWh and Celectrical - €0.160/kWh.

It is appropriate to evaluate both terms (Equations 10.21 and 10.22) for any time interval (month, season, year). Whenever NES or NCS become zero or less, the operation of a ventilation system with HR should be shut down or by-passed, respectively.

Since Celectrical/Cthermal — 3.56 is different from and actually always greater than PEelectrical/PEthern;all which is in between 2.18 (— 2.4/1.1) and 2.73 (— 3.0/1.1) depending on all-European or national German conditions, the value of NCS can be zero or less even if a positive primary energy balance according to Equation 10.21 still does exist.

It is recommended to evaluate the economic value NCS (Equation 10.22) as a criterion instead of the energy-related quantity NES (Equation 10.21). This describes the common practice to make decisions in a more realistic way. NCS can be used to decide on operational conditions for a given ventilation system, as well as for comparisons between different system options.

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