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Design and analysis of an integrated heat and energy recovery ventilation system with economizer control for net-zero energy solar houses.

INTRODUCTION

The growing energy crisis and the growth of research and finding in both medical and technological fields have increased the awareness on energy recovery and also the need of proper ventilation in closed environments. With improper ventilation in homes the built up of excess moisture causes toxins, pollutants and gases, nurturing the growth of mold and dust mites. Therefore, mechanical ventilation systems such as HRV and ERV have been introduced to recover latent and sensible energy loss within homes. An HRV is normally designed for cooler climates to heat the cool fresh air using indoor warm air through a sensible heat recovery core. The ERV, on the other hand, is beneficial for dry and polar climates where moisture retention is crucial and tropical climates, where moisture removal is vital to reduce latent cooling load. As well, certain weather conditions (like spring or fall where minimal heating/cooling is required) may only require an economizer mode, which enables fresh air to be pumped into the home without any conditioning.

As North America is one of the largest energy consumers, they have taken steps to moderate this issue in various ways (Natural Resources Canada, 2000). Due to Canada's cold and dry climate, one third of the country's energy is consumed in the building sectors (Marrone, 2007). Therefore, residential heating and cooling is a major part of this energy usage. In order to reduce the energy demand, modern homes are increasingly constructed to be air tight and mechanical systems such as HRV and ERV are used for providing the required ventilation to maintain an acceptable level of indoor air quality. Hence, the purpose of HERV is an integration of both HRV and ERV with an economizer feature.

OBJECTIVE

The main objective of this project is to design and build a prototype of a system which includes; HRV, ERV, and Economizer, for the North America market. This system is called the HERV, which will integrate the benefits of each of the discussed systems as well as providing a smart automated control system. The human control interface will enable the user to control the level of fresh air input into the system and other parameters of the AHU. From the outside temperature and relative humidity inputs, the HERV system will determine whether the system should run on the economizer, HRV, ERV or both the HRV and ERV mode. Based on which mode is required, the control system will open and close the appropriate dampers and turn on and off the appropriate fans.

Design Requirements:

* Proposed Markets: Regions in North America where there are ranges of temperature from hot to cold, and conditions from humid to dry

* Efficiency: Maximize energy efficiency by reducing energy consumption and frictional losses

* Size: The Heat and Energy Recovery Ventilation System (HERV) must be relatively small and compact similar to most HRV and ERV systems on the market

* Simplicity: The system has to be user friendly and operable by homeowners

METHODOLOGY

This project aims to use two methodologies to test the HRV/ERV/Economizer device -- one an experimental approach, the other -- TRVE (A customized Excel based program for the evaluation of the potential benefits of the proposed HERV system in different climatic conditions). The experimental method will be used once the device is manufactured during the winter of 2012. In order to do the experimental analysis the device will be installed in a super energy efficient TRCA Archetype house (Zhang et al., 2011) in Toronto.

The TRVE program methodology, however, is an analytical approach based on general HVAC formulas and climatic condition of the region considered for the analysis. This program is of a particular benefit because based on the energy savings obtained using the TRVE program for a specific region the user may or may not opt in for installing the combined HRV/ERV/HERV device. Figure 1 illustrates the logic of and input and output of the TRVE program.

[FIGURE 1 OMITTED]

The program is a Microsoft Excel Spreadsheet based tool that determines the heat loss in the house based on the hourly weather data of the climatic location; building footprint; the extent of airtightness; and exterior envelope information, including exterior walls, roof, windows, basement floor and underground walls and their U-values. The tool uses the Bin Method principles for load calculations instead of the hourly load analysis used by softwares such as HAP, eQuest, EE4, and HOT2000. Further, the tool takes the performance of the building's furnace and air conditioning into account for load calculations. It should be noted that the internal heat gains have also been accounted for; including the heat load from occupants and the electrical appliances, which are based on NRCan's R-2000, EnerGuide standards and are included in the HOT2000 analysis tool as well. The tool also accounts for the ventilation requirements, based on the ASHRAE Standard 62.2-2010: Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings, and compares the performance of the HERV system against a traditional ERV or an HRV system.

RESULTS AND DISCUSSION

After conducting the preliminary analysis for various zones across North America using the developed TRVE tool, the annualized average efficiencies of the proposed HERV system were obtained. It is clear from the Table 1 that the combined HRV/ERV (HERV) is almost always as efficient as individual HRV or ERV alone except for the Tropical/Megathermal Climates. The reasoning is due to the HERV's functionality, where it has the capability to switch between different modes; sensible recovery (when there is a minor difference in the relative humidities between indoors and outdoors), latent recovery (when there is a minor difference in the temperatures between indoors and outdoors); Sensible & Latent (when there is a significant difference in both temperatures and relative humidities between indoors and outdoors); and the economizer control (when both temperature and relative humidity variations are minor). Due to this control system that evaluates the difference between the indoor and outdoor setpoints (temperature and humidity), higher savings are achieved compared to a traditional ERV or an HRV. Thus, the HERV system offers ease of control and the capability to operate under different conditions and at the same time maintain required thermal comfort within the house.
Table 1: Weighted annual efficiencies for each system in
different climatic zones

Zones      Type of Climates       HERV     ERV      HRV
                                 System  System  System

Zone   Tropical/Megathermal         38%     31%     77%
A      Climates

Zone   Dry Climates                 61%     58%     73%
B

Zone   Temperate/Mesothermal        53%     20%     72%
C      Climates

Zone   Continental/Microthermal     59%     57%     69%
D      Climates

Zone   Polar Climates               62%     63%     63%
E

Note: The efficiencies shown for the HRV system represent
sensible efficiency only, while the HERV and ERV systems
show a combined Sensible & Latent efficiency.


Furthermore, the annual operating savings, shown in Table 2, clearly indicates that a HERV system is always beneficial in any type of climate. Table 3 indicates that the polar climates have the shortest payback period of 13 years.
Table 2: Annual savings for each system in different
climatic zones

Zones    Type of Climates     HERV     ERV       HRV
                             System  System   System

Zone  Tropical/Megathermal       $7    -$156    -$43
A     Climates

Zone  Dry Climates              $171     $45     $30
B

Zone  Temperate/Mesothermal      $75     $59     $34
C     Climates

Zone  Continental/Microthermal  $193     $71     $76
D     Climates

Zone  Polar Climates            $282    $203    $125
E

Table 3: Payback periods for each system in different
climatic zones

Zones       Type of Climates        HERV      ERV      HRV
                                  System   System   System

Zone A  Tropical/Megathermal         487       No       No
        Climates                          Payback  Payback

Zone B  Dry Climates                  21       32      130

Zone C  Temperate/Mesothermal         54       No       44
        Climates                          Payback

Zone D  Continental/Microthermal      20      223       17
        Climates

Zone E  Polar Climates                13        8        9


It should be noted that the determined efficiencies of the HERV system are based on the individual hourly performance data of the ERV and HRV in the TRCA Archetype house in Toronto. This, however, will be revised based on the testing and final experimental results for the device. These will be obtained once the device is manufactured and proper tests are performed, which will tentatively be performed before May 2012.

CONCLUSION AND ONGOING WORK

The HERV system is a new concept and hopefully this will be a breakthrough in the HVAC industry, which will lead to greater and more efficient energy conservation. North America is one of the highest energy consuming countries in the world, and the benefit from this concept could lead to further research and development in energy efficient homes. The HERV system combines the benefits of both ERV and HRV systems, allowing it to maximize sensible and latent energy recovery.

A virtual prototype of the HERV was created and evaluated using the developed spreadsheet program called TRVE. From the preliminary analysis, it was recommended that a HERV be used for dry climates (Zone B), continental/microthermal climates (Zone D) and possibly polar climates (Zone E) due to their expected annual energy savings. HERV would be beneficial in these climates due to its energy savings and short payback period. However, its potential high incremental cost may be a disadvantage in certain cities resulting in unacceptably high payback periods.

Further research and development will be conducted for the proposed HERV system. Experimental results for the proposed HERV system will be obtained once the final design and prototype are created and properly tested at the Ryerson Campus Labs as well as the TRCA Archetype House in Toronto.

Benefits of the proposed HERV System

The following is the schematic/ physical layout of the proposed HERV system--Most of the heat recovery systems available on market are either HRV or ERV or ERV without economizer controls. However it has been found that if HRV used along with ERV has benefits and can be beneficial for a particular weather conditions. When the temperature difference between the indoor and the outdoor is greater than 2[degrees]C and the relative humidity difference is greater than 5% there is a requirement for both sensible and latent heat transfer. ERV by itself can accomplish this particular task, however, ERV cores are less efficient then the HRV cores. Hence it is useful if the indoor air is allowed to exchange heat with the outside air and then the indoor air is allowed to exchange both heat and humidity with the outside air as show in the schematic above. This way the efficiency for heat and moisture transferred is increased because of the use of both the HRV and ERV in series.

[FIGURE 2 OMITTED]

Furthermore, the sensible efficiency of the HRV core is given by (Zhang, L-Z., 2008)

[[epsilon].sub.sensible] =(m [c.sub.p])f([T.sub.2]) - [T.sub.1] / (m [c.sub.p])f([T.sub.2] - [T.sub.1]) (Sensible efficiency)--Eq. 1

The latent efficiency of the ERV core is given by--

[[epsilon].sub.latent] =(m [c.sub.p])f([w.sub.2]) - [w.sub.1]/ (m [c.sub.p])f([w.sub.2] - [w.sub.1]) (Latent efficiency)--Eq. 2

And the combined efficiency is given by--

[[epsilon].sub.overall] = [e.sub.sensible] + [e.sub.latent].H*/(1 + H) (Overall efficiency)--Eq. 3

Where

T1: Temperature of outside air before the heat exchanger ([degrees]C)

T2: Temperature of outside air after the heat exchanger ([degrees]C)

T3: Temperature of inside air before the heat exchanger ([degrees]C)d

w1: Humidity ratio of outside air before the heat exchanger (kg/kg)

w2: Humidity ratio of outside air after the heat exchanger (kg/kg)

w3: Humidity ratio of inside air before the heat exchanger (kg/kg) mf: Mass flow rate of fresh air

[m.sub.e]: Mass flow rate of indoor air

H*= 2501 [DELTA]w/[DELTA]t

Using this above formulation and test data for separate efficiency of the HRV and ERV cores combined efficiency for the above system can be calculated. Based on this efficiency payback periods and annual savings were calculated as seen in Table 2 and 3 for the combined HERV system. As seen the combined HERV system has benefits over individual HRV or ERV.

ACKNOWLEDGMENTS

The authors would like to thank ASHRAE for providing the funding for this student capstone project through its Senior Undergraduate Project Grant Program.

NOMENCLATURE

ACH = Air Changes per Hour

AHU = Air Handling Unit

ERV = Energy Recovery Ventilator

HVAC = Heating, Ventilating and Air Conditioning

HERV = Heat and Energy Recovery Ventilator

HRV = Heat Recovery Ventilator

TRCA = Toronto and Region Conservation Authority

REFERENCES

American Society of Heating, Refrigerating and Air-Conditioning Enginners. (2009). 2009 ASHRAE Handbook -- Fundamentals. American Society of Heating, Refrigerating and Air-Conditioning Engineers

Barua, Rupayan, Zhang, Dahai, and Fung, Alan S. (2010). Analysis of Energy Performance of the Sustainable Archetype House at Kortright Centre. 1st International High Performance Buildings Conference, West Lafayette, Indiana, USA, July 12-15, 2010.

Dembo, Aya, Ng, Ka Long Ringo, Pyrka, Agatha, and Fung, Alan. (2010) The Archetype Sustainable House: Investigating its potentials to achieving the net-zero energy status based on the results of detailed energy audit. 1st International High Performance Buildings Conference, West Lafayette, Indiana, USA, July 12-15, 2010.

Marrone, J. (2007). Getting to Zero: Defining the Path to Net Zero Energy Home Construction. Natural Resources Canada.

Natural Resources Canada. Office of Energy Efficiency. (2000). Demand Policy and Analysis. Ottawa.

Rupayan Barua, D. Z. (2010). Implementation of TRCA Archetype Sustainable House Monitoring System. Department of Mechanical and Industrial Engineering, Ryerson University.

Toronto Region Conservation Authority. (2009, Feburary 7). Archetype Sustainable House. Retrieved December 19 2011, from TRCA Kortright Center: http://www.sustainablehouse.ca/rentals/vanEE. (n.d.). How VanEE Air Exchanger's Work. Retrieved December 20 2011, from VanEE: http://www.vanee.ca/hwGvaneeGairexchanger.php

Zhang, Dahai, Barua, Rupayan, and Fung, Alan S. (2010). Development of Monitoring System for the Sustainable Archetype House at Kortright Centre. 1st International High Performance Buildings Conference, West Lafayette, Indiana, USA, July 12-15, 2010.

Zhang, Dahai, Barua, Rupayan, and Fung, Alan S. (2011). TRCA-BILD Archetype Sustainable House--Overview of Monitoring System and Preliminary Results for Mechanical Systems. ASHRAE Transactions, 117(2), 2011, pp. 597-612.

Zhang, L.-Z. (2008). Total Heat Recovery: Heat and Moisture Recovery from Ventialtion Air. Nova Science Publisher, Inc.

Carl Yu Chen

Student ASHRAE Member

Jiten Mistry

Student ASHRAE Member

Alan S. Fung, PhD, Peng

ASHRAE Member

Wey H. Leong, PhD, PEng

ASHRAE Member

Sumeet Jhingan EIT, LEED[R] AP

ASHRAE Member

Carl Y. Chen and Jiten Mistry are BEng students in the Department of Mechanical Engineering, Ryerson University, Toronto, Ontario. Alan S. Fung and Wey H. Leong are associate professors in the Department of Mechanical Engineering, Ryerson University, Toronto, Ontario. Sumeet Jhingan is a Building Energy & Sustainability Analyst with Morrison Hershfield Limited.
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Author:Chen, Carl Yu; Mistry, Jiten; Fung, Alan S.; Leong, Wey H.; Jhingan, Sumeet
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:1CANA
Date:Jul 1, 2012
Words:2419
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