Exergy-optimum morphing air-to-air heat recovery ventilation

Abstract: This paper introduces an axially morphing heat recovery ventilation unit. This unit addresses the thermodynamic knowledge gaps in the current theory and practice without the recognition of the Second Law of thermodynamics, and the fixed design geometry and control parameters used. To avoid these gaps, an axially and laterally morphing heat recovering system has been modelled and analysed. The model analyses the decarbonization effect. Sample analyses show that such a design may benefit up to 150% increase of carbon-dioxide reduction potential when electric power from a grid-connected, natural-gas power plant is supplied, compared with a conventional design with fixed geometry, based on the First Law only.

Keywords: Morphing HRV, Exergy, Nearly avoidable CO₂, Green Building Metric

Introduction

Today, buildings are responsible for 15.3 Gton of CO₂ emissions per year. Despite such a high emission rate, green building metrics are limited by the First Law of Thermodynamics, whereas nearly avoidable emissions, ∆CO₂ due to exergy mismatches between the energy supply and demand, may be as high as seven times the direct emissions measured or predicted by the First Law [1, 2]. Air-to-air heat recovery systems for ventilation of buildings (HRV) is one of them. Current standards and guidebooks do not relate First Law efficiency and effectiveness directly to emissions. For example, in AHRI Standard 1061-SI-2018, mass and ‘energy’ inequalities are the only testing and rating factors. In this respect, the sensible energy inequality is a function of mCp∆T, and the resulting CO₂ emissions are not described, whereas if an exergy imbalance would be considered, then the exergy imbalance, namely mCp∆T(1-To/T1), according to the ideal Carnot Cycle would result in ∆CO₂, according to the Equation 1 [3]:

The proportionality factor, k is either 1.1 or 2.1, depending upon where the major exergy destruction occurs. It is 1.1, if it takes place downstream of the useful application, like a solar PV panel (destroys thermal exergy after generating electric power), and 2.1, if the useful application like HRV takes place after the utilization of power (electrical input to the fans). Figure 1 shows an air-to air heat recovery in ventilation (HRV), without recirculation of the exhaust air, as described in ASHRAE Handbook S 26 an S 41.4 [4]. According to the First Law of Thermodynamics, the coefficient of performance, COP is given in Equation 2, which does not recognize the relation between exergy destructions, temperatures, and ∆CO₂.

Figure 1. Sensible Heat Exchanger, HRV [4, 5].

Figure 2. New Exergy Flow Diagram of HRV. Exergy Destruction are Ignored by Current Models.

Method

An axially morphing HRV is considered (Figure 3). It has flexible, accordion type connectors to optimize the effective length, Li according to the varying operating conditions and the fresh air demand. The amount of thermal power, Q to be unity (unit heat) for the sake of simplicity of the analysis. The objective of axial morphing and change of the heat transfer surface is to maintain a negative ∑CO₂ by controlling the fan power with the constraint given in Equation 3.

Figure 3. Axially Morphing Sensible Heat Exchanger, HRV with Variable Li.

Figure 4 shows that part of the exhaust air at a ratio of X is mixed with the fresh outdoor air to conserve energy, either in heating or cooling. In this case, latent exergy of the exhaust air and the outdoor supply air needs to be considered. This model excludes additional fan power for mixing (PR). Figure 5 shows a two-axis-morphing case.

Figure 4. Recirculating Air Case in Energy Recovery 0 ≤x ≤ L.

Figure 5. Laterally and Axially (Bidirectional) Morphing HRV.

Figure 6. Variation of ∑CO₂ Performance of an Axially Morphing HRV with Different Electric Supply.

Sample result and discussion

This research showed that even a seemingly simple system like HRV becomes overly complicated from the environmental perspective beyond the First Law of thermodynamics. For example, even the question where the electric power becomes (energy mix) becomes an important design and on-line HRV control issue, when the matter is CO₂ emission calculations. A bidirectionally morphing HRV unit seems to be analytically solvable and digitally implementable in the field for sustainable buildings. All green building metrics are based on the First Law only [2]. Theoretically, if one installs a conventional HRV unit in a so-called green building, the outcome in most cases may be ineffective or adverse, if all the exergy-components of design and operation of HRV are not considered and implemented.

Symbols

CO₂                     Direct emission, kg CO₂/kW-hen

COP                     Coefficient of performance

CP                        Specific heat, kW-hen/(kg·K)

k                           Emission responsibility factor, kg CO₂/(kW-hex/kW-hen)

L                           Axial effective HRV length, m

m                          Mass, kg

P                           Power demand of a fan (including motor efficiency), kW

Q                          Thermal energy (Heat) or thermal power, kW-hen, kWen

T                           Temperature, K or °C

x                           Axial distance from the supply air side to indoors (Figure 5)

Greek Symbols

ε                           Unit exergy, kW-hex/kW-hen

ψR                        REMM efficiency

ηI                          First-law efficiency

∆CO₂                   Nearly avoidable CO₂ emission responsibility, kg CO₂/kW-hen

ΣCO₂                   Total emissions, ∆CO₂+CO₂, kg CO₂/kWen

∆T                        Temperature difference, K

Subscripts

des                       Destroyed (exergy)

e                           Exit (Exhaust air)

en                         Energy

ex                         Exergy

o                           Outdoor, original

PP                        Power plant

R                          Recirculation (Fan)

ref                        Reference

s                           Supply

Acronyms

HB                       Handbook

HRV                    Heat recovery ventilation

NG                       Natural Gas

PV                        Photovoltaic (Panel)

PVT                     Photovoltaic-thermal (Panel)

References

[1]     Kilkis, B. 2019. Decarbonization: Exergy to The Rescue, REHVA J., 2019-3, pp: 24-30. https://www.rehva.eu/rehva-journal/chapter/decarbonization-exergy-to-the-rescue

[2]     Erten D., Kılkış B. 2022. How Can Green Building Certification Systems Cope with The Era of Climate Emergency and Pandemics? Energy and Buildings 256: 111750. DOI: 10.1016/j.enbuild.2021.111750.

[3]     Kılkış, Ş., and Kılkış, B. 2024. Rational Exergy Management Model Based Metrics For Minimum Carbon Dioxide Emissions And Decarbonization In Glasgow, Energy Journal (in print).

[4]     ASHRAE. 2000. ASHRAE Handbook HVAC Systems and Equipment Volume, Chapter 26, Air-To-Air Energy Recovery Equipment, Atlanta, GA, USA.

[5]     ASHRAE. 2025. ASHRAE Handbook Fundamentals Volume, Chapter 37 (Tentative), Cognizant Chapter: TC 7.4, Exergy for Sustainable Buildings, Exergy Rationale For Sustainable Buildings and Decarbonization (in revisor stage), Atlanta, GA, USA. (in revisor stage).