Scholarly article on topic 'Solar Thermal Trigeneration System in a Canadian Climate Multi-unit Residential Building'

Solar Thermal Trigeneration System in a Canadian Climate Multi-unit Residential Building Academic research paper on "Earth and related environmental sciences"

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{"Solar Thermal" / "Thermally Driven Cooling" / "Thermally Driven Heat Pump" / Cogeneration / "Multi-Residential Building" / "Cold Climate"}

Abstract of research paper on Earth and related environmental sciences, author of scientific article — Martin Kegel, Justin Tamasauskas, Roberto Sunye

Abstract This paper presents the comparison of several solar thermal, cogeneration and thermally driven heating/cooling central plant systems implemented into a typical mid-rise apartment, located in Calgary, Alberta, Canada. TRNSYS is used to model the system and predict the primary and secondary energy consumption, greenhouse gas (GHG) emissions and annual utility costs of the various systems and compared to the base case. The highest annual utility cost and GHG emission savings was attained by operating a cogeneration device in priority in a solar thermal, cogeneration with absorption heat pump plant, predicting a 21% reduction in the annual utility cost and a 16% reduction in GHG emissions. The system however has a higher secondary energy consumption, 10% above the base case. Operating the solar thermal collectors in priority over the cogeneration unit in the same central plant the greatest primary and secondary energy savings were attained achieving 16% and 18% savings compared to the base case. GHG emission savings of 16% was predicted with both operating strategies. Comparing this system to a cogeneration only system, secondary energy savings of 36% are predicted demonstrating the benefit of adding solar thermal collectors and a thermally driven heat pump to a cogeneration system. Extending the operating period of the solar thermal and cogeneration system by adding a thermally driven chiller did not yield significant primary energy, secondary energy, GHG emission or utility cost savings over a heating only system. This is attributed to extended operating period of the single stage boiler with poor part load performance. Finally, due to the low natural gas utility costs and higher electricity rates, the systems operating the cogeneration system in priority always resulted in the highest utility cost savings and the addition of solar thermal and thermally driven heating/cooling had limited economic benefit.

Academic research paper on topic "Solar Thermal Trigeneration System in a Canadian Climate Multi-unit Residential Building"

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Energy Procedia 48 (2014) 876 - 887

SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry

September 23-25, 2013, Freiburg, Germany

Solar thermal trigeneration system in a Canadian climate multi-unit

residential building

Martin Kegela*, Justin Tamasauskasa, Roberto Sunyea

aNRCan-CanmetENERGY-Varennes, 1615 BlvdLionel Boulet, Varennes, QC, Canada

Abstract

This paper presents the comparison of several solar thermal, co generation and thermally driven heating/cooling central plant systems implemented into a typical mid-rise apartment, located in Calgary, Alberta, Canada. TRNSYS is used to model the system and predict the primary and secondary energy consumption, greenhouse gas (GHG) emissions and annual utility costs of the various systems and compared to the base case. The highest annual utility cost and GHG emission savings was attained by operating a cogeneration device in priority in a solar thermal, cogeneration with absorption heat pump plant, predicting a 21% reduction in the annual utility cost and a 16% reduction in GHG emissions. The system however has a higher secondary energy consumption, 10% above the base case. Operating the solar thermal collectors in priority over the cogeneration unit in the same central plant the greatest primary and secondary energy savings were attained achieving 16% and 18% savings compared to the base case. GHG emission savings of 16% was predicted with both operating strategies. Comparing this system to a cogeneration only system, secondary energy savings of 36% are predicted demonstrating the benefit of adding solar thermal collectors and a thermally driven heat pump to a cogeneration system. Extending the operating period of the solar thermal and cogeneration system by adding a thermally driven chiller did not yield significant primary energy, secondary energy, GHG emission or utility cost savings over a heating only system. This is attributed to extended operating period of the single stage boiler with poor part load performance. Finally, due to the low natural gas utility costs and higher electricity rates, the systems operating the cogeneration system in priority always resulted in the highest utility cost savings and the addition of solar thermal and thermally driven heating/cooling had limited economic benefit.

© 2014 TheAuthors.Published by ElsevierLtd.

Selectionandpeerreview bythe scientific conferencecommitteeofSHC 2013underresponsibilityofPSE AG

Keywords: Solar Thermal; Thermally Driven Cooling; Thermally Driven Heat Pump; Cogeneration; Multi-Residential Building; Cold Climate

* Corresponding author. Tel.: 1-450-652-4823; fax: 1-450-652-5177. E-mail address: martm.kegel@mcan-nrcan.gc.ca

1876-6102 © 2014 The Authors. Published by Elsevier Ltd.

Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.101

1. Introduction

The Canadian residential sector accounts for 16% of the national secondary energy consumption, where space heating, space cooling and domestic hot water production represent over 80% of the residential energy end use [1]. While improvements in the minimum levels of insulation have reduced the energy required for space heating, this end use continues to be the dominant consumer, accounting for over 60% of the total energy consumption. Furthermore, an increase in demand to maintain comfort levels during the summer has seen cooling energy consumption double over the past 17 years (1990 to 2007). Finally, with new buildings continuously being constructed, electrical grids are becoming strained, and thus energy conservation measures reducing space heating, space cooling and electrical loads are becoming increasingly important.

Previous studies have shown that small scale trigeneration systems (combustion engine with absorption chiller) implemented into residential single dwelling houses can significantly reduce the primary energy consumption as well as greenhouse gas emissions when coal fired electricity production is displaced [2]. While promising results were attained, the small scale trigeneration system lacked an economy of scale, resulting in long payback periods for the homeowner, and required the on-site electricity production not used to be fed back into the grid, not always desirable by utility companies. Furthermore, there was a large increase in the secondary energy consumption (77%) by producing electricity onsite through the natural gas fired combustion engines. To improve homeowner payback periods, it was proposed to investigate larger dwellings where the services can be shared and to incorporate absorption or adsorption heat pumps used to meet space heating and cooling demands thereby extending the operation of the thermally driven system and reducing the size of the cogeneration unit [2]. While the use of solar energy to produce electricity is becoming increasingly popular in Canada, there is still a large untapped potential to use this energy for space heating. To do so, however, large storage tank reservoirs are required often making the system unfeasible. By combining solar thermal with a similar trigeneration system previously studied for space heating, space cooling and domestic hot water (DHW) production, a practical storage tank size can be installed while having a reasonable solar fraction.

To investigate the viability of a solar assisted trigeneration system in a heating dominated climate, this paper presents an analysis performed in TRNSYS on a typical mid-rise apartment building located in Calgary, Alberta, Canada that meets the current national energy building code. The province of Alberta presents a unique opportunity for solar assisted trigeneration, as 49% of the electricity produced is through coal fired power plants [3], and there is an abundance of solar energy available (means daily horizontal insolation: 3.3 - 4.2 kWh/m2) [4].

2. Energy model development

The mid-rise apartment model used for the analysis was based of the Department of Energy (DOE) archetype energy models [5] adapted to the Canadian National Energy Code for Buildings (NECB), 2011 [6]. The mid-rise apartment is four stories with 8 apartment suites on each floor. The south-east apartment suite on the main floor is assumed to be a rental office and thus the mid-rise apartment is comprised of 31 rental apartment units and one office. Each floor also has a shared corridor. The total building floor area is 3,135 m2, with a footprint of 784 m2. The mid-rise apartment archetype model was developed in TRNSYS for its strength in modeling non-standard HVAC systems, which was the main focus of this study. Each apartment was modeled as a separate zone with the 2nd floor and 3rd floor apartments combined to reduce model complexity. In total, the model is comprised of 27 zones. Characteristics include wall RSI values of 4.81 m2-°C/W, roof RSI values of 6.21 m2-°C/W, double glazed, vinyl framed, low-e windows with a centre of glass U-value of 1.98 W/m2-°C and shading coefficient of 0.46. Each apartment is ventilated through a heat recovery ventilator with an effectiveness of 0.5 bringing in 32 L/s of fresh air. Space heating and cooling in each suite is accomplished through a packaged terminal air conditioning (PTAC) unit with hydronic heating coil. The central heating plant is comprised of two single stage gas fired boilers operating in parallel with a thermal efficiency of 83%. The boiler setpoint temperature is varied with the NECB 2011 recommended outdoor air reset control strategy (maximum 82.2°C to a minimum of 60°C set point temperature). The secondary heating loop was modeled with single speed variable flow pumps. Primary and secondary pumping powers were set to 300 W/(L/s) as per ASHRAE 90.1 [7]. Flow through each apartment suite's hydronic heating coil is modulated to meet the space heating demand controlled by a central thermostat located in each apartment suite. A

supply air temperature between 21°C and 43°C was typically maintained during the heating season. The air conditioning cooling capacities for each apartment were selected to meet the calculated sensible cooling load. Actual cooling coil sizes were selected according to manufacturer available sizes - typically between a 2.6 kW and 5.3 kW (9,000 to 18,000 BTU/hr) total cooling capacity providing a 13°C supply air temperature during the cooling season. Airflows for the apartment PTAC units were selected according to manufacturer recommended flow rates and fan powers were set to 4.2 W/(L/s) as per ASHRAE 90.1 [7]. To ensure comfort conditions are met, an electric steam humidifier was added in each suite, ensuring the relative humidity in each apartment remains above 30% during the dry winter months. During the summer, the relative humidity never exceeds 60%, with dehumidification accomplished through the cooling unit. Lighting and receptacle loads were modeled according the NECB 2011 default values. Half the apartments were assumed to have an occupancy of 2 while the other half an occupancy of 3 people. Space temperatures were maintained at 21°C during the heating season and at 23°C during the cooling season. DHW is stored and heated in a natural gas fired central 1,000 L DHW tank with a 40 kW heating capacity, maintained at 60°C. Each occupant is assumed to draw 70 L of hot water per day, slightly less than that published in the IEA/ECBCS Annex 42 daily domestic hot water consumption per occupant in Canada [8]. Additional details of the archetype model can be found in the paper by Tamasauskas et al. [9]. A breakdown of the predicted annual energy end use is shown in Fig. 1a below with a plot of the monthly heating, DHW and cooling loads shown in Fig 1b. The annual heating, DHW and cooling loads are 675 GJ, 300 GJ and 85 GJ, respectively.

Fig 1: (a) Mid-rise apartment energy end use breakdown and (b) Mid-rise apartment monthly heating, DHW and cooling load

The annual electricity and natural gas consumption, estimated annual utility rates and estimated greenhouse gas emissions are summarized in table 1. The annual utility costs are provided to demonstrate the relationship between energy and utility cost savings in Canada. The annual utility costs were taken from ENMAX Energy Corporation [10]. Each apartment unit was assumed to be individually metered and tenants paid the regulated rate estimated at slightly more than $0.12 CDN/kWh. The shared services (corridors and rental office) were assumed to be metered under the same residential rate. Natural gas was assumed to be under the medium commercial rate (D300) and the monthly costs shared among all apartment units. A floating natural gas rate was assumed with the 2012 historical monthly rates used [11]. Natural gas cost was around $5.00 CDN/GJ. Greenhouse gas emission factors for electricity and natural gas were taken from Farhat and Ugursal [3]. To estimate the primary energy consumption for electricity production, 4% transmission losses were assumed [3] and a 30.23% power plant efficiency as reported in the Electric Power Generation, Transmission and Distribution Report from 2007 by Statistics Canada [12]. A primary energy factor of 1.1 for natural gas was also used to take into account the energy required for gas refinement [13]. The total annual secondary energy consumption is 2,575.0 GJ or 0.82 GJ/m2.

Table 1: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) Annual GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 1,053.3 $43,802 227.8 3,623.8

Natural Gas 1,521.7 $6,859 77.1 1,673.9

Total 2,575.0 $50,661 304.9 5,297.7

3. System 1: Solar thermal integration

The first step in assessing the feasibility of the novel trigeneration system was to implement the solar thermal collectors into the energy model. The solar collectors would be mounted on the roof, facing south and tilted at 51° (the latitude of Calgary) because of the intended summer and winter use. Evacuated tube collectors were chosen because of high temperature requirements for the thermally driven heat pump system to be evaluated. Vitosol 300 SP3 evacuated tube collectors were selected with the performance characteristics taken from the Directory of SRCC Certified Solar Collector Ratings [14]. Ensuring there is no shading on the collectors, a total of 40 collectors were proposed to be installed on the roof for a total collector area of 172 m2. A 5,000 L storage volume was selected, as the higher temperature fluid energy would eventually be injected into a lower temperature storage tank. The tank is maintained up to a 90°C temperature and the solar collector pumps are activated if the solar collector outlet temperature is 2°C higher than the tank temperature. In the event there is insufficient solar energy to meet the space heating requirements, two single stage, 83% efficient boilers are installed with each other and in series with the solar storage tank and controlled with the same outdoor air reset controller as in the base case. The storage tank can be bypassed in the event the return temperature is greater than the storage tank temperature. To increase the use of solar energy during the summer, the solar collectors are also used for DHW preheating. An auxiliary heater is used to ensure this DHW tank temperature remains above 60°C. A schematic of the system is shown in Fig. 2.

Fig 2: Proposed solar thermal system schematic

Results of the annual energy consumption, annual utility costs, estimated annual GHG emissions and annual primary energy are summarized in table 2. The percentage change compared to the base case model is also reported.

Table 2: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after solar thermal integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 1,058.4 (+0.5%) $43,980 (+0.4%) 228.9 (+0.5%) 3,641.4 (+0.5%)

Natural Gas 1,158.0 (-23.9%) $5,446 (-20.6%) 58.7 (-23.9%) 1,273.8 (-23.9%)

Total 2,216.4 (-13.9%) $49,426 (-2.4%) 287.6 (-5.7%) 4,915.2 (-7.2%)

By implementing the evacuated tube solar thermal collectors, natural gas savings of 23.9% annually are predicted, amounting to approximately $1,235 in annual utility cost savings. As anticipated, with the low natural gas rates and high costs associated with evacuated tube solar collectors, this system would most likely not have a viable payback period. The actual payback period and a life cycle cost analysis will be calculated in future work. The solar fraction of the system, defined as the percentage of solar energy used to meet the space heating, cooling and DHW loads is 36.1%.

To improve the system energy savings, a larger storage volume could be implemented, but is not always desirable. This would also further increase the cost of an already expensive system. Alternatively, cogeneration systems have shown promise in Canada, using the low natural gas rates to meet space heating and electricity production needs.

4. System 2: Cogeneration integration

With the low natural gas rates and electricity rates costing close to seven times more per unit of energy in Alberta, cogeneration systems can be an economically viable system. Furthermore, producing electricity onsite, primary energy and GHG savings can be achieved by offsetting electricity produced by the coal fired power plants and transmission losses. To avoid the scenario where electricity would be sold back to the grid, which is not always desirable by the utility company, the cogeneration system was sized to meet the average daily electrical load of the shared services and apartment lights and receptacles estimated at 16 kW. This estimated load was also below the minimum electrical draw of the midrise apartment calculated to be 16.8 kW. The cogeneration unit is operated to follow the thermal load of the building generate the maximum power. Future work will look into operating the cogeneration system to follow the electrical load of the building. In the event the maximum power production of the cogeneration system exceeds the building electrical demand, the engine is operated at part load.

The thermal and electrical efficiency of a cogeneration unit is dependent on the coolant fluid temperature, coolant flow rate and net power production. This performance data is only available through extensive testing, and thus the cogeneration system selected was based on the AISIN 6 kWe (kW electric) unit, previously tested for a Natural Resources Canada Project by Advanced Engine Technology [15]. At full load conditions, the selected cogeneration system has an electrical efficiency of 26.7%, thermal efficiency of 47.9% resulting in a total system efficiency of 74.6%. The model assumes three AISIN 6 kWe units are installed, which has a full load electrical output of 17.6 kW and thermal output of 32.6 kW.

The cogeneration system was modeled using Type 154 developed by the International Energy Agency (IEA) Annex 42 [16]. This type maps the performance of the cogeneration unit (electrical and thermal efficiency) with the coolant flow rate, coolant entering temperature and electrical loading. The performance map is described by a 27-term tri-variate equation. Additional details of the TRNSYS type can be found in the IEA Annex 42 report: Specifications for Modelling Fuel Cell and Combustion-Based Residential Cogeneration Devices with Whole-Building Simulation Programs [16].

In the TRNSYS model, the cogeneration system is connected to a 10,000 L storage tank, installed in series with the back-up boiler. The cogeneration unit was controlled to maintain an 82°C tank outlet temperature, the maximum temperature recommended by the cogeneration manufacturer. In the event there is insufficient energy from the cogeneration device to meet the space heating requirements, one single stage, 83% efficient boiler is installed in series with the cogeneration storage tank and controlled with the same outdoor air reset controller as in the base case. The 10,000 L storage tank could be bypassed in the event the return temperature is greater than the tank outlet

temperature. To operate the cogeneration system during the summer months, the DHW storage tank was also fed from the 10,000 L storage tank. The DHW tank was equipped with a back-up heater in the event the tank temperature falls below 60°C. A schematic of the system is presented in Fig. 3.

Fig 3: Proposed cogeneration system schematic

Table 3 summarizes the results implementing a cogeneration only system into the mid-rise apartment. The percentage change to the base case is also indicated.

Table 3: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after cogeneration only integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 651.4 (-38.2%) $29,830 (-31.9%) 140.9 (-38.1%) 2,241.0 (-38.2%)

Natural Gas 2,663.5 (+75.0%) $11,674 (+70.2%) 135.0 (+75.1%) 2,929.9 (+75.0%)

Total 3,314.9 (+28.7%) $41,503 (-18.1%) 275.9 (-9.5%) 5,170.9 (-2.4%)

As anticipated, there is significant decrease in electricity purchased from the onsite conversion of natural gas to electricity. While this system results in 18.1% annual utility cost savings, the total secondary energy consumption increases by 28.7%, and the primary energy consumption decreases close to 2.5%. Comparing the cogeneration system to the solar thermal system (system 1), the primary and secondary energy savings by system 1 exceed the cogeneration unit, however it is unable to achieve the same level of annual utility cost savings. Thus, the utility cost savings obtained through a cogeneration system and the secondary energy savings attained by a solar thermal system could result in an interesting combination.

5. System 3: Solar thermal and cogeneration integration

To evaluate the viability of a solar thermal, cogeneration system, both systems modeled individually were combined into one model. Solar energy collected by the 172 m2 array would be stored in a 5,000 L storage tank farm and injected into a 10,000 L storage tank. Heat is also injected into the 10,000 L storage tank through the 17.6 kWe cogeneration system. The space heating loop and building DHW system are fed from the 10,000 L storage tank. The space heating loop is equipped with a back-up boiler in the event there is insufficient energy to meet the space heating load, and an auxiliary DHW heater ensures the tank temperature never falls below 60°C. A schematic of the system is shown in Fig. 4.

The system is operated such that energy production through solar is given priority (system 3a). Energy is transferred from the 5,000 L storage tank to the 10,000 L storage tank whenever beneficial. In the event the

10,000 L storage tank supply temperature falls below 70°C, the cogeneration unit is fired. To meet peak heating loads, a single stage, 83% efficient boiler is installed in series after the 10,000 L storage tank and controlled with the same outdoor air reset controller as in the base case. The storage tank temperatures are limited to maximum 90°C.

Fig 4: Proposed solar assisted, cogeneration system schematic

The predicted results are shown in table 4. The percent savings/increases to the base case system are also indicated.

Table 4: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after solar thermal and cogeneration integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 895.9 (-14.9%) $38,329 (-12.5%) 193.7 (-15.0%) 3082.2 (-14.9%)

Natural Gas 1,654.3 (+8.7%) $7,681 (+12.0%) 83.9 (+8.8%) 1,742.3 (+4.1%)

Total 2,550.2 (-1.0%) $46,009 (-9.2%) 277.6 (-9.0%) 4,824.5 (-8.9%)

Secondary energy and primary energy savings compared to the cogeneration only system (system 2) were attained by implementing the 172 m2 solar array. However, due to the reduced operation of the cogeneration system, by giving the solar system operating priority, the annual utility cost is $4,500 more per year and GHG emissions remained approximately the same. Although energy savings are attained, the results do not demonstrate an economic benefit to installing solar thermal collectors with the cogeneration system. The solar fraction of the system increases only slightly (to 36.2%) compared to the solar only system, although the total storage volume increased to 15,000 L. This is attributed to the cogeneration system implementation, where cogeneration energy would be injected in the event the storage tank outlet temperature falls below 70°C. Thus there are instances where there is no benefit to operating the solar collector because no energy gain can be attained due to the high tank temperature. As a check, the 15,000 L tank system energy model was run without the cogeneration system and resulted in a solar fraction of 37.7%. As anticipated the solar fraction of the system increased with a larger storage tank volume; however not the extent expected. The small increase in solar fraction can likely be attributed to the non-coincident heating loads and solar energy availability and thus although slightly more energy can be stored, the high demand during the colder and darker periods are still greater than the storage capacity.

To evaluate an alternative control strategy of the above system, instead of operating the solar thermal collectors in priority, priority is given to the cogeneration system to maintain an 82°C tank temperature (system 3b). In the event the supply temperature falls below 70°C, solar energy from the 5,000 L storage tank is injected. A single stage

back-up boiler with 83% thermal efficiency is installed in series to ensure the required heating loop supply temperature is attained. The results are summarized in table 5. The percentage difference in primary and secondary energy consumption, utility costs and GHG emissions compared to the base case is also indicated.

Table 5: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after cogeneration with solar thermal integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 682.4 (-35.2%) $30,907 (-29.4%) 147.6 (-35.2%) 2,347.6 (-35.2%)

Natural Gas 2,287.0 (+50.3%) $9,955 (+45.1%) 115.9 (+50.3%) 2,515.7 (+50.3%)

Total 2,969.4 (+15.3%) $40,861 (-19.3%) 263.5 (-13.6%) 4,863.3 (-8.2%)

By operating the cogeneration system in priority over the solar collectors (system 3b), the secondary energy consumption is 15.3% higher than the base case compared to 1.0% below running the solar collectors in priority (system 3a). Producing more electricity, the annual utility costs are lower as are the estimated GHG emissions and primary energy consumption; however the secondary energy consumption is still higher than the base case. Furthermore, the large solar thermal system only reduces the annual utility costs by $700 in comparison to the cogeneration only system (system 2). The solar fraction of system 3b is estimated to be only 11.1%, not optimizing the use of the solar thermal collectors. To improve the annual utility cost savings, the same system is evaluated running the solar thermal system in priority; however an absorption chiller is implemented to increase the use of thermal energy during the summer, and offsetting the electricity demand of the PTAC systems in each suite.

6. System 4: Solar thermal and trigeneration integration

A previous study investigated the techno-economic feasibility of implementing a thermally driven chiller in residential houses [2] driven by a cogeneration unit (small scale trigeneration system). While promising primary energy savings (35%) were attained, along with homeowner utility costs (61%), the residential houses failed to have a significant cooling load to overcome the high cost associated with a small scale thermally driven chiller. With the mid-rise apartment having a larger cooling load than a single family dwelling, this scenario evaluates the benefit of replacing the PTAC units in each apartment with a 2 pipe fan coil connected to a trigeneration plant. A schematic of the system is shown in Fig. 5.

Fig 5: Proposed solar assisted, trigeneration system schematic

The thermally driven system was sized for a peak daily cooling load of 22.5 kW occurring on July 19. A Yazaki WFC-SC-30 absorption H2O/LiBr chiller was selected [17], which has a 22.5 kW cooling capacity at a generator inlet temperature of 70°C, cooling water inlet temperature of 31°C, chilled water outlet temperature of 7°C and the fully rated heat medium flow ratio. To reduce the required pumping power, the generator flow was modeled at half the rated flow. At the de-rated flow and same operating conditions, the absorption unit has an estimated cooling capacity of 19.5 kW and a thermal COP of 0.5. The power consumption of the absorption chiller is 310 W, not including the heat rejection system and circulation pumps. Since reducing the secondary and primary energy consumption is the main objective of the system, the solar system was given operating priority over the cogeneration system (system 4a). A back-up boiler ensures the minimum 70°C absorption chiller supply temperature is maintained. The boiler is also used to ensure the space heating loads are met and controlled through an outdoor air reset controller. To avoid low part load operation scenarios with the absorption chiller, a 1,000 L chilled water storage tank was modeled (not shown in Fig. 3). The absorption chiller is operated to maintain a 7°C tank temperature. The results are summarized in table 6. The percent savings to the base case are also highlighted.

Table 6: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after solar thermal, cogeneration and absorption chiller integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 876.1 (-16.8%) $37,641 (-14.1%) 189.5 (-16.8%) 3,014.1 (-16.8%)

Natural Gas 1,691.6 (+11.1%) $7,781 (+13.4%) 85.7 (+11.1%) 1,860.8 (+11.1%)

Total 2,567.7 (-0.3%) $45,422 (-10.3%) 275.2 (-9.7%) 4,874.9 (-8.0%)

By replacing the PTAC units with a 2 pipe fan coil system served by a solar thermal trigeneration central heating and cooling plant, primary energy, secondary energy, GHG emission and utility cost savings were attained compared to the base case. Furthermore, the solar fraction of the system increases to 39.5% by utilizing solar energy to meet the space cooling load. In comparison with the solar thermal cogeneration plant (system 3 a), there is only slight improvement in the savings of all four categories. This is attributed to the increased electricity consumption of the additional pumping power required for the absorption system and chilled water loop as well as the extended operating period of the back-up boiler running at poor part load efficiency. In addition, because of the defined heating and cooling season for the 2 pipe fan coil season, there is a period in early September, where space heating is required, however the demand is not met, because the system still operates in cooling mode. To evaluate an alternative control strategy the same system as above is evaluated; however the cogeneration unit is run in priority (system 4b). The results are summarized in table 7 with the percentage change to the base case indicated.

Table 7: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after cogeneration, solar thermal and absorption chiller integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 651.8 (-38.1%) $29,843 (-31.9%) 141.0 (-38.1%) 2,242.4 (-38.1%)

Natural Gas 2,498.3 (+64.2%) $10,950 (+59.6%) 126.6 (+64.2%) 2,748.1 (+64.2%)

Total 3,150.1 (+22.3%) $40,793 (-19.5%) 267.6 (-12.2%) 4,990.5 (-5.8%)

With the cogeneration system operated in priority (system 4b), improved utility cost savings are predicted over operating the system with solar thermal in priority (system 4a) similar to the conclusions arrived with the previous systems. Comparing the results to the cogeneration, solar thermal system only (system 3b), there is again only a slight benefit to adding an absorption chiller ($70 annual utility cost savings). Furthermore, system 3b has better secondary energy, primary energy and GHG emission savings than the absorption system evaluated (system 4b). Future work will investigate alternative thermally driven systems (such as ejectors or adsorption chillers) as well as evaluating the system with a fully modulating or condensing back-up boiler which have improved part load efficiencies over a single stage boiler.

One of the additional drawbacks associated with thermally driven chiller is the short operating period. To extend the operation of a thermally driven system, an absorption heat pump is proposed capable of operating during the heating and cooling periods with a beneficial thermal COP.

7. System 5: Solar thermal and trigeneration integration with heat pump

This scenario evaluates the potential of replacing the absorption chiller from the previous scenario with an absorption heat pump (system 5a). While indirect fired thermally driven chillers with a heat pump mode operation were not found to be commercially available, a literature review showed that laboratory tests conducted on an absorption chiller demonstrated that it can also be operated in heat pump mode with a beneficial COP [18]. To evaluate the impact and/or benefit of such a system, the performance data of the absorption heat pump was extrapolated and applied to the proposed solar thermal, cogeneration, absorption chiller system (system 4). The system operates on the principle of using a driving heat source (from CHP and solar) and a low temperature heat source (ambient, down to -2°C) to produce a medium heating supply temperature (around 40°C). The system has a thermal COP of 1.5 at a 40°C supply temperature, 95°C inlet generator temperature and 10°C inlet evaporator temperature. As supply temperatures increase and/or generator and evaporator temperatures decrease the thermal COP decreases as well. The low temperature heat source can be supplied by the ambient air or more typically, from boreholes storing the heat rejected from the absorption heat pump during the cooling season. Due to the cold Canadian winter, boreholes storing the heat rejected from the cooling season are proposed as the use of ambient air as a low temperature heat source would not be suitable. The boreholes are sized to store the heat rejected from the system during the cooling period estimated to be 280 GJ. With an annual heating load of 975 GJ and a design COP of 1.3, the amount of low grade heat required is 225 GJ. Thus, the dry fluid cooler installed in parallel with the boreholes is used during the summer period to ensure the borehole injection and extraction loads are balanced. A diagram of the system is shown in Fig. 6. To store 225 GJ of heat rejection, sixteen boreholes of 100 m in length were calculated with a spacing of 6.1 m. By adding the boreholes, the heat rejection temperature in cooling is lower, thereby improving the efficiency of the absorption system in cooling mode as well.

Fig 6: Proposed solar assisted, trigeneration system schematic with absorption heat pump (heating mode)

Due to a lower heating supply temperature (40°C instead of 82°C) compared to the other systems (systems 1 -4), additional heating coil rows are added to the fan coil (increase from one to two rows to four rows) and the design fluid flow rate to meet the heating loads is increased approximately three times, to ensure comfort conditions can be

met with a lower heating supply temperature. Results are summarized in table 8 with the percentage savings of the primary, secondary energy, utility costs and GHG emissions to the base case indicated.

Table 8: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption and comparison to the base case after solar thermal, cogeneration and absorption heat pump integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 899.3 (-14.6%) $38,448 (-12.2%) 194.5 (-14.6%) 3,093.9 (-14.6%)

Natural Gas 1,216.5 (-20.1%) $5,707 (-16.8%) 61.7 (-20.0%) 1,338.2 (-20.1%)

Total 2,115.8 (-17.8%) $44,155 (-12.8%) 256.2 (-16.0%) 4,432.1 (-16.3%)

It can be seen that greater primary energy, secondary energy, GHG emission and utility cost savings are incurred in comparison to the thermally driven cooling system (system 4a) only. While electricity consumption increases due to the increased pumping power, the absorption heat pump reduces natural gas consumption by over 28% compared to an absorption chiller system (system 4a) highlighting the benefit of thermally driven heat pumps. Furthermore, there is a 36.2% reduction in the secondary energy consumption compared to the cogeneration only central plant (system 2). Taking into account the reduced thermal energy required to meet the space heating loads through the use of the heat pump, the system has a solar fraction of 46.2%. The use of an absorption heat pump with solar thermal collectors and a cogeneration system is predicted to provide the best combination of primary energy and secondary energy savings in comparison to all systems assessed. An analysis operating the same system with cogeneration priority (system 5b) is summarized in table 9.

Table 9: Estimated annual electricity and natural gas consumption, utility costs, GHG emissions and primary energy consumption with a comparison to the base case after cogeneration, solar thermal and absorption heat pump integration

Service Secondary Energy Consumption (GJ) Utility Cost ($, CDN) GHG emissions (ton CO2 eq.) Primary Energy Consumption (GJ)

Electricity 676.5 (-35.8%) $30,703 (-29.9%) 146.3 (-35.8%) 2,327.5 (-35.8%)

Natural Gas 2,151.6 (+41.4%) $9,485 (+38.3%) 109.1 (-29.3%) 2,366.8 (+41.4%)

Total 2,828.1 (+9.8%) $40,188 (-20.7%) 255.4 (-16.2%) 4,694.3 (-11.4%)

As anticipated, the greatest utility cost savings are attained with this type of system and control strategy as well as the highest GHG emission savings, as the system has greatest reduction in net building electricity consumption; however the secondary energy consumption increases by 9.8% compared to the base case or 33.7% compared to operating the system in solar thermal priority (system 5a).

8. Conclusions, recommendations and future work

An assessment was performed on various combinations of solar thermal, cogeneration and thermally driven heating and cooling central plant systems for a typical mid-rise apartment located in Calgary, Alberta, Canada. The evaluation was performed in TRNSYS comparing the annual primary and secondary energy consumption, annual utility costs and annual greenhouse gas emissions for each of the proposed systems. It was predicted that the highest utility cost and GHG emission savings compared to the base case is achieved operating the cogeneration device in priority in the solar thermal, cogeneration with absorption heat pump central plant system, achieving 21% annual utility cost savings and 16% GHG emission savings. Operating the same system in solar thermal priority the highest primary and secondary energy savings were achieved compared to the base case predicting a 16% and 18% reduction, respectively. Limited benefit was found in implementing a thermally driven cooling system to extend the solar thermal and cogeneration operating period, as keeping an inefficient boiler online during the summer months and the increased pumping power resulted in limited annual utility cost savings. Furthermore, due to the low natural gas utility rates, little benefit was found in adding solar thermal collectors and thermally driven systems to a cogeneration only system. The benefit in adding these systems was that the secondary and primary energy

consumption is reduced by 36% and 12%, respectively. While the objective of the paper was to present an energy consumption comparison of the various systems, the annual utility cost savings highlight that these types of systems are better suited for larger community scale projects, where a high system cost can be distributed over several houses and buildings. From the point of view of system simplicity (and thus likely apparent investment cost), the cogeneration only system is the most suitable establishing 18% annual utility cost savings; however such a system also has close to a 29% increase in secondary energy consumption - not ideal if utility costs increase. Future work is planned to perform a 20 year life cycle on these systems in addition to evaluating them on a larger scale. It is also planned to investigate alternative thermally driven systems, such as ejectors or adsorption units, more efficient backup boiler systems (condensing or fully modulating boilers), implementing improved performance data for thermally driven heat pump systems, evaluating the systems with a radiant floor, comparison to electrically driven heat pump systems, alternative cogeneration control strategies and tank temperature set points as well as performing a sensitivity analysis on the utility rates.

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