1. Introduction
Small combined heat and power (CHP) systems or so-called micro-cogeneration are able to generate power and heat at the point of use by utilizing a variety of conventional and renewable technologies. Micro-cogeneration systems (with an electric output lower than 50 kW
el according to [
1]) are emerging as a suitable approach to reduce energy consumption and pollutant emissions by offsetting the need for centrally-generated grid electricity, enhancing energy security and avoiding transmission/distribution losses [
2,
3,
4,
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26]. In recent years, significant advances have been achieved in the development, design, and optimization of micro CHP systems such as micro-turbine [
6,
7], internal combustion engine [
8,
9,
10,
11,
12,
13], organic Rankine cycle [
6,
14,
15], Stirling engine [
8,
16,
17,
18], fuel cell [
7,
19,
20,
21,
22,
23,
24,
25] and photovoltaic thermal (PVT) [
8,
19,
26].
In addition to the micro-cogeneration technologies, several studies have recognized that the ground source heat pump (GSHP) is an efficient and environmentally friendly option for space heating/cooling of buildings [
27,
28,
29,
30]. A large number of GSHPs have been used in residential and commercial buildings throughout the world [
30]. The GSHP performance is still a subject of on-going research efforts, since it is affected by physical parameters (e.g., heat reservoir temperature and geological characteristics of the site), operation characteristics (e.g., the time distribution of the cooling/heating demand), as well as construction and qualitative characteristics of the system [
29,
30]. Lee et al. [
31] explored an optimization method that factors the impact of ICT (information and communication technologies) on improving the GSHP performance. Several other studies [
32,
33,
34,
35] have investigated energy, cost and environmental performance of trigeneration systems (heating, cooling and power) that integrate water-to-water GSHP with fuel cell and PVT technologies respectively.
Ground-to-air heat exchange (GAHX) is one of the technology options for green buildings that utilize various concepts for natural heating, ventilation and air-conditioning [
36]. GAHX system is usually used in colder countries in Europe and America [
37] for cooling and heating. GAHX takes advantage of constant ground temperatures to precondition incoming fresh air hence reducing the heating and cooling loads. The earth’s surface ground temperature remains relatively constant at a depth of 1.5–2.0 m [
38]. When the fresh air passes through the GAHX pipelines, it is pre-cooled with cooler ground temperature in the summer and pre-warmed with the ground heat in winter by using the thermal potential of the ground. The energy required for cooling or heating the building reduces significantly by using GAHX system when the difference between outdoor temperature and the comfortable indoor temperature is small. In addition, the use of GAHX downsizes the heating and cooling equipment capacities.
To further reduce the energy consumption, new initiatives have been launched around the world. Passive house has been introduced in European countries such as Germany and Switzerland to enhance the low energy buildings concept [
39]. Renewable heat obligation (RHO) has been implemented in Republic of Korea for public sector buildings larger than 10,000 m
2 to reduce heating energy consumption by at least 11% between 2015 and 2030 [
40]. The passive house standard calls for air tightness ≤0.6 air change per hour (ACH) and maximum space heating load for residential building ≤15 kWh/m
2·year [
41]. Therefore, the passive house can be equipped with geothermal systems which integrating heat pump with a ground-to-air heat exchanger to meet the cooling and heating load demands.
Chel et al. [
42] simulated multi-zone building to investigate the thermal performance of integrated air-air heat exchanger (AAHE) and earth-water heat exchanger (EWHE) for a passive house. They concluded that AAHE and EWHE systems were able to provide 72% reduction in annual heating consumption with an energy intensity of 6.9 kWh/m
2·year, which was within the passive house standard. The effect of the earth tube heat exchanger on the energy saving for a small house, row houses and a small office building was investigated under three different climate conditions [
42]. The results from their simulations showed increased energy saving between 2.2 and 9.4 kWh/m
2 for the small house, between 1.3 and 4.1 kWh/m
2 for the row houses, and 0.7 kWh/m
2 for the small office building. The geothermal system is able to save 43% and 37% of the primary energy consumption during the heating and cooling seasons, respectively [
43].
Photovoltaic thermal (PVT) is another technology that can be integrated into microgeneration systems [
33,
34,
35]. The dual functions of the PVT system result in a higher overall solar conversion rate than that of solely photovoltaic or solar collectors, thus enabling a more effective use of solar energy. Currently, the number of commercially available collectors and systems are still very limited. Major barriers such as product reliability and costs remain to breakthrough [
44,
45], but PVT devices are expected to have major market expansion potentials in the near future [
44]. Significant activities are still required in terms of PVT energy conversion and its effectiveness, thermal absorber design and fabrication, material and coating selection, performance testing, cost minimization, system optimization, control and reliability [
44,
45].
Although micro CHP, GSHP and PVT are exciting technologies, they are still facing several challenges, in gaining shares in mature and competitive markets, in further improving of devices efficiency and reducing cost, in increasing the operational lifetime to recover the initial investment, and also in understanding the systems both by installers and potential end users.
High purchase cost is the main barrier for larger penetration of the above-mentioned technologies in residential applications. Most of these technologies are in an early stage of commercialization and they do not benefit from the economies of scale in comparison to other large renewable parks. However, the recent technology advances in PV cells led to dramatic cost reduction of the solar panels that would positively affect the PVT penetration in both residential and commercial markets. The newly introduced government and utility incentive programs are also expected to rapidly accelerate the wide spread diffusion of renewable and advanced microgeneration technologies.
Annex 54 of the International Energy Agency’s Energy in Buildings and Communities Programme (IEA/EBC) has undertaken in-depth analysis of microgeneration and other energy technologies [
46]. The Annex 54 includes, among many research activities, study of multi-source micro-cogeneration systems, polygeneration and renewable hybrid energy systems, and their analysis when serving single and multiple residences along with small commercial premises [
47,
48].
Smart building-integrated trigeneration technology meets many countries government’s new research and development policies about future growth engine items: (1) new and renewable hybrid engines; (2) shift from unit to integrated technology; (3) change from source side to demand side systems and (4) from centralized macro to decentralized micro technologies. Recognizing the importance of trigeneration systems for reducing the CO2 emissions in the future energy mix, Korea Institute for Energy Research and CanmetENERGY Research Centre have initiated a joint project to investigate renewable trigeneration applications in residential and commercial buildings in Korea and Canada. The component technologies considered in the trigeneration system include air-to-water heat pump (AWHP), ground-to-air heat exchanger (GAHX) and photovoltaic thermal (PVT). The proposed system combines the three stand-alone renewable technologies into one system under common control. The technologies are integrated in a way that they can function alone or in any combination to meet the imposed load. By implementing this approach the demand will be met in an optimal way utilizing the best mix of renewable sources available at the time.
This paper discusses system performance, integration issues and investigates the energy and environmental impacts of the proposed renewable trigeneration system in a residential building application under Incheon (Korea) and Ottawa (Canada) weather conditions. The two cities were chosen because: both cities are quite conducive to the use of PVTs, ground source heat pump systems and renewable energies; the project teams have research facilities and also have access to experiment data in these two locations, which are essential for validating the simulation models both at component and system levels.
2. Renewable Trigeneration System for Investigation
Two systems (cases), with conventional and hybrid renewable microgeneration technologies, were developed and analyzed for residential building applications. Case one is a conventional boiler-chiller system and Case two is the hybrid GAHX-PVT-AWHP trigeneration system. Both systems are able to meet the requirements of a single detached house for its space heating, space cooling and domestic hot water heating.
Through the first case, the residential thermal loads are met with a chiller and boiler system and a fan coil unit as presented in
Figure 1. The fan coil unit is located inside the house and a duct system is used to distribute the cooling/heating air. Domestic Hot Water (DHW) tank is installed inside the house and connected to the boiler via pipelines for DHW supply.
The second case, the renewable trigeneration system as shown in
Figure 2, uses an air-to-water heat pump (AWHP) to satisfy the house heating and cooling load demands. The AWHP is integrated with a horizontal ground-to-air heat exchanger (GAHX) and a photovoltaic thermal (PVT) system. The GAHX, sometimes also called earth-tube, utilizes the thermal mass of the soil surrounding to preheat or precool the ambient air before entering to the AWHP, and thereby to increase the efficiency of the heat pump and reduce energy consumption. The PVT panels are built as modules by manufacturers and can generate both electric and thermal energy. The generated electric energy can be used to operate the AWHP both in heating and cooling seasons thereby reducing the electric power import from the grid to the house. The generated thermal energy is used to preheat inlet air to the AWHP in the heating season.
A hot water storage tank is equipped to store energy for house space heating and DHW heating. In summer, the chilled water from the AWHP flows directly to the cooling coil unit without using a cold water storage tank. Three way valves are used to switch between AWHP heating and cooling loops in winter heating and summer cooling seasons, as shown in
Figure 2. An electric heater is installed to provide DHW heating in summer, and also in winter to supplement DHW heating in cases where the energy stored in the storage tank is insufficient. The city water has enough pressure to flow the water in the system without using a pump in the DHW loop.
4. Simulation Models and Assumptions
4.1. TRNSYS Models
TRNSYS software platform (Version 17.02) [
57] is used in the present study to simulate the trigeneration system and the reference conventional system described in the previous section. It is one of the most advanced dynamic building energy simulation programs. TRNSYS library includes a large database of component models (which are called “Type” in TRNSYS) related to buildings, thermal and electrical energy systems, input and output data management and other dependent functions [
58]. The components can be connected to form complex systems. In the present study, component models were selected from the TRNSYS libraries and enhanced with latest manufacturers’ systems performance data. Summary of the TRNSYS types used in the simulation and verification of the reference system model was presented in Entchev et al. [
32]. Brief descriptions are given to the major components of the trigeneration system, i.e., ground-to-air heat exchanger, PVT panels and air-to-water heat pump in the following.
Type 997: multi-level horizontal ground heat exchanger was used to model the ground-to-air heat exchanger. A series of horizontal pipes are buried in the ground to transfer heat from/to the ground. There are different pipe configurations such as parallel, serpentine, double-serpentine and intertwined quadruple serpentine. The impact of the energy storage in the ground is considered where the pipes are surrounded with 3-D rectangular conduction model. An experimental test-rig has been setup in a field in Korea (as shown in
Figure 3), and testing data collected at various operating conditions will be used to further validate the TRNSYS GAHX model in the next phase of the study.
The PVT panels were modeled by Type 560: PV/T collector interacting with simple zone models. This model relies on linear factors relating the efficiency of the PV cells to the cell temperature and also the incident solar radiation. Performance data obtained from a manufacturer was used to validate the model. The model outputs (electric/thermal energy and efficiency) are in good agreement with the manufacturer’s data with relative errors within 10%.
The air-to-water heat pump was modeled using TRNSYS Type 941. It is not a first principles model, but relies instead upon catalogue data readily available from heat pump manufacturers. The component requires two data files, one for cooling performance and another for heating performance. Both data files provide capacity and power draw of the heat pump as a function of entering water temperature and entering air temperature to the heat pump.
4.2. House Specifications and Thermal Loads
The hybrid trigeneration system is assumed to serve a one-story residential house with a floor area of 200 m
2 and a height of 2.7 m. The building specifications meet the building envelope requirements for climate zone 4 recommended by ASHRAE Standard 90.1–2007 [
59].
Space heating and cooling loads are dependent on the house geographic location, building envelope and room thermostat set-point. These loads will be presented later in the results section. The house domestic hot water profile is based on the Canadian Centre for Housing Technology (CCHT) [
60] simulated DHW-draw profile and average hot water usage value (252 L/day) recommended by ASHRAE [
59] for “typical” families. The DHW consumption for the house used in the present study is 255 L/day with 7 draws. Usually the DHW usage pattern is different in weekdays and weekends, however, the same profile is used in the simulation for simplicity.
4.3. Equipment Capacity and Specifications
In the reference case, the boiler is a high-performance condensing boiler with a capacity of 30 kW for Ottawa case and 25 kW for Incheon case. The boilers have a steady state efficiency of 94%. The chiller has a capacity of 3-ton for Ottawa case and 4-ton for Incheon case, and a rated coefficient of performance (COP) of 3 under testing conditions (entering water temperature 7 °C and ambient air temperature 35 °C). The hot water tank for DHW storage has a volume of 190 L (50 US gallons).
For the trigeneration system, the ground-to-air heat exchanger is assumed to have a total length of 250 m with tube inner diameter of 0.25 m. The tube is buried at 2 m deep in serpentine configuration. There are 10 PVT panels installed vertically on south-facing façade. Two PVT panels are connected in series, which forms 5 parallel rows. Each PVT panel has a rated electric capacity of 295 W and thermal capacity of 1535 W. The air-to-water heat pump has a rated heating capacity of 9 kW with a COP of 3.3 under testing conditions (entering air temperature 7 °C and water temperature 40 °C), and a rated cooling capacity of 8 kW with a COP of 2.8 under testing conditions (entering air temperature 35 °C and water temperature 12 °C). The storage tank volume is 1000 m3.
4.4. Control Approaches
The room temperature is controlled based on the schedule presented in
Table 2 for heating and cooling season respectively.
The control strategies for the reference system and the trigeneration system are shown in
Table 3 and
Table 4 respectively.
For the reference system, the boiler is turned on when there is a call-for-heat signal either from the room thermostats or the DHW storage tank aquastat. The boiler outlet temperature is controlled at 82 ± 2 °C and a tempering valve reduces the hot water temperature supplied to the heating coils down to 60 °C. The temperature in the domestic hot water storage tank is controlled by two separate temperature nodes (T
2 is located in the second tank node that is near the tank top and T
5 located in the middle of the tank) as shown in
Table 3. Whereas the chiller on/off is controlled by the room thermostat set-points in the cooling season and the chilled water temperature is kept at 7 ± 2 °C.
The air-to-water heat pump operation in the trigeneration system (Case 2) is controlled by the aquastat (s) near the bottom (T
8) of the hot water tank as shown in
Table 4. The AWHP is turned on when T
8 ≤ 40 °C and turned off until T
8 ≥ 45 °C. Since the aquastat T
8 is located at the bottom of the storage tank, the temperature at the tank top is expected to be typically in the range between 45 and 50 °C.
The ambient air is generally preheated by the ground-to-air heat exchanger before entering the AWHP in the heating period. However, if the average air temperature inside the PVT panels is 5 °C higher than the air near the outlet of the earth tube, the preheated air from the PVT is drawn to the AWHP until the temperature difference is less than 2 °C. In cooling season, the ambient air is precooled by the GAHX before entering the AWHP.
The electric heater for DHW heating is turned on when the DHW temperature from the hot water storage tank is less than 49 °C. It should be noted that the DHW heating is solely provided by the electric heater in the summer as the AWHP is in cooling mode.
4.5. Weather Data and Simulation Period
The system performance is not only depending on its configuration, but also depending on its geographic location, which affects space heating/cooling load profiles. As mentioned earlier, two cities were chosen for evaluating the performance of the proposed trigeneration system: Incheon, Korea and Ottawa, Canada. The weather data source for Ottawa is Canadian Weather for Energy Calculations (CWEC) database, and for Incheon is International Weather for Energy Calculations (IWEC) database. Both sets of weather data were downloaded from EnergyPlus website [
61] in EnergyPlus format.
In Incheon, the warmest month is in August with a daily average of 25.2 °C and the coolest month is January with a daily average of −2.1 °C. The annual average temperature is 12.1 °C. In Ottawa, the warmest month is in July with a daily average of 20.5 °C and the coolest month is January with a daily average of −11.5 °C. The annual average temperature is 6.3 °C.
For Incheon, the heating season is assumed to start from mid-October to April with the cooling season from the beginning of May to mid-October. However, for Ottawa, the cooling season is slightly shorter, starting from June to end of September due to its relatively cool summer compared to Incheon.