Waste Heat Recovery Technologies on Optimized CHP-BESS Plant: A Performance Comparison Between Organic Rankine Cycle and H2O-NH3 Absorption Plant

The last decade has highlighted how climate change is deteriorating the biological and economic sustainability of many sectors worldwide. Continuously shifting and irreversible weather patterns influences the integrity of ecosystems in numerous ways, including fluctuations in species abundance, changing in distribution ranges, alterations in activity patterns, and variations in microhabitat utilization. Additionally, climate change is increasing antimicrobial resistance, posing a significant threat to human health by amplifying the incidence of resistant pathogenic infections. Concurrently, diseases transmitted through various vectors such as water, food, and air, as an example is the coronavirus pandemic, are on the rise [1]. Inside the health sector the sensibleness for the risks linked to the climate change is increasingly growing, not only due to its impact on the human health but also because it represents one of the major polluters, responsible for 5.2% of global emissions [2]. Policy priorities [3, 4] play a critical role inside healthcare sector, aimed to balance the supporting investments in low-carbon technologies with the one in medical facilities, where pervasive unmet basic healthcare needs persist alongside underdeveloped healthcare systems [5]. This work presents a techno-economic comparison between two technologies for waste heat recovery from an optimized combined heat and power (CHP) plant, integrated with a battery energy storage system (BESS), aimed at minimizing fossil primary energy (PES). The comparison is based on the hourly energy demand of the Italian Hospital facility CROB (Oncological Reference Center of Basilicata).

During the last years the role played by cogeneration plants inside the healthcare sector is of primary importance, ensuring energy demand during electric grid or renewable plants faults, while reducing emissions and energetic costs. Gimelli et al. [6] implemented a genetic algorithm based - vector optimization methodology [7] to determine the optimal size of a modular CHP-BESS plant (Figure 2) for the CROB Hospital, aimed to maximize the PES index in comparison to the conventional separate production of equivalent energy. The electric power produced by the natural gas internal combustion engines, in cogeneration asset, covers the Hospital’s electric demand, which exceeding power is stored inside the BESS or sold to the local grid; the thermal power coming from the exhaust gases and from the coolant water and lubricant oil, covers the Hospital’s thermal demand. Anytime the CHP-BESS plant is not able to meet the required load of the Hospital, the natural gas boiler is turned on and the electric power is imported from the local grid. According to the PES hourly evaluation, a proper engine’s on/off control strategy has been implemented in the work. During a specific time interval, the decision to turn on or off the engines is based on the fossil primary energy saving performed by the plant to meet Hospital’s hourly demand. In fact, every time the PES>0, the engines are turned on, assuring a fossil primary energy saving; if the PES<0, the engines are off and the Hospital’s demand is covered by the separate production of energy. Although CHP plants can work at partial loads [8], with relative several electric and thermal power output, this study has been based on the assumption that partial loads are not taken in account. So, the gas engines operate only at the nominal point. The details of optimal solution are reported in Table 2, the plant is characterized by three ICEs, each one delivering 443 kW of electric power at the nominal working point; the total recoverable thermal power is 523.56 kW, which 68% (356 kW) comes from the exhaust gases (at the constant temperature of 350℃), the remaining 32% (168 kW) comes from coolant water and lubricant oil (at the constant temperature of 90℃). The system is coupled with a small BESS, with 68 kWh of capacity.

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Figure 2. Optimized CHP-BESS plant scheme

Table 2. Characteristics of the optimized CHP-BESS system

The logic integration between the thermodynamic models of the ORC plant and APC system within the optimized CHP-BESS model is shown in Figure 5. For every hour of the year the Hospital’s electric, thermal, and cooling demand, together with the economic values assumed as constant, represent the main input for the numerical models. Every time the CHP plant is on, after covering the Hospital’s electric and thermal demand, the remaining engines’ exhaust gases thermal power is recovered inside the ORC plant for the production of electric power, integrated inside the hourly Hospital’s electric demand, stored inside the BESS, or sold to the local grid. When the engines’ exhaust gases thermal power is recovered into the APC system, a proper control strategy, based on the hourly Hospital’s electric and cooling demand, splits the ammonia vapor between the electric and the cooling production line, always aimed to prioritize the cooling power production rather than the electric one, accordingly to the APC system performances in converting thermal power to cooling power with higher efficiency compared to the conversion efficiency in electric power. The cogenerated electric and thermal power is then integrated respectively with the external electric local grid and auxiliary boiler, in order to meet the hourly energetic Hospital’s demand. At the end of the calculation, the main outcomes are represented by the PES index, the reduction of CO₂ emissions, and the SPB period, used to compare the performances of the proposed systems.

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Figure 5. Workflow of the integrated CHP-BESS thermodynamic model with ORC and APC models

The Eq. (1) for the PES index takes into account all the energy flows occurring between the Hospital, the proposed integrated plant, and the auxiliary systems (electric grid and thermal boiler).

         $P E S=\frac{E_{P, R S}-E_{P, P S}}{E_{P, R S}}$           (1)

The fossil primary energy of the reference systems (separated production of the energy) ($E_{P, R S}$) is represented by Eq. (2).

$E_{P, R S}=E_{P, T H}+E_{P, E L}+E_{P, E L E X C}$            (2)

It is given by the sum of the fossil primary energy respectively required by the thermal boiler ($E_{P, T H}$) and the electric local grid ($E_{P, E L}$), in order to cover Hospital’s thermal and electric demand, and the fossil primary energy of the exceeding electric energy ($E_{P, E L E X C}$) related to the proposed integrated plant. The Eq. (3) shows in detail the terms of the fossil primary energy for the reference system:

$E_{P, R S}=\frac{E_{T H}}{\eta_{T H, r e f}}+\frac{E_{E L}}{\eta_{E L, r e f}}+\frac{E_{E L, E X C}}{\eta_{E L, r e f}}$          (3)

where, $E_{T H}$ represents the yearly Hospitals’ thermal energy demand; $\eta_{T H, r e f}$ is the reference efficiency for the thermal boilers, assumed equals to 0.90; $E_{E L}$ represents the yearly Hospital’s electric energy demand; $E_{E L, E X C}$ is the yearly exceeding electric power related to the proposed systems; $\eta_{E L, r e f}$ represents the reference efficiency of the local electric grid, assumed equals to 0.45.

The fossil primary energy of the proposed system ($E_{P, P S}$), shown in Eq. (4).

$E_{P, P S}=E_{P, C H P}+E_{P, T H_{\text {int }}}+E_{P, E L_{\text {int }}}$           (4)

It is obtained through the sum of the fossil primary energy required by the CHP system ($E_{P, C H P}$), and the sum of the fossil primary energy related to the integrated thermal energy ($E_{P, T H_{\text {int }}}$) and integrated electric energy ($E_{P, E L_{i n t}}$) respectively from the thermal boiler and electric local grid, in order to meet Hospital’s demand. The Eq. (5) shows in detail the terms of the fossil primary energy for the proposed system:

$E_{P, P S}=m_c H_i+\frac{E_{T H, I N T}}{\eta_{T H, r e f}}+\frac{E_{E L, I N T}}{\eta_{E L, r e f}}$        (5)

where, $m_c H_i$ is the product between the yearly fuel mass required by the cogeneration system and the fuel lower heating value; $E_{T H, I N T}$ represents the thermal energy integrated during the year by the thermal boiler to cover the Hospital’s demand; $E_{E L, I N T}$ is the electric energy integrated during the year by the local electric grid, to cover the Hospital’s electric demand.

The economic comparison among presented integrated plants is based on the SPB evaluation, calculated as shown in Eq. (6).

$S P B=\frac{I C}{\Delta C_{t o t}}$            (6)

The IC represents the investment cost of the proposed systems, reported in detail with Eq. (7):

$I C={Cost}_{C H P}+{Cost}_{B E S S}+{Cost}_{W H}$          (7)

where, ${Cost}_{C H P}$ and ${Cost}_{B E S S}$ represent the investment cost of the optimized CHP-BESS solution [6]; ${Cost}_{W H}$ is the investment cost of the waste heat plants considered in this work: the ORC [12] or the APC [21] system. The investment costs are summarized in Table 5.

Table 5. Investment costs of the proposed systems

The $\Delta C_{\text {tot }}$ constitutes the annual cost saving, expressed with the Eq. (8).

$\Delta C_{t o t}=C_{t o t_{R S}}-C_{{t o t_{P S}}}$           (8)

It is expressed as the difference between two aliquots: $C_{ {tot }_{R S}}$ is the total energy cost of the reference system (separate production of the same amount of energy), containing the annual electric and thermal energy required by the Hospital; $C_{ {tot }_{P S}}$ represents the total energy cost for the proposed system, considering the annual cost of the cogeneration’s gas methane and the integrated electric and thermal energy respectively from the electric local grid and auxiliary boiler. In Table 6, the average cost of the gas methane, the imported and exported electricity, are shown based on the Italian pricing system.

Table 6. Reference value of the energetic costs for the Italian pricing system [6]

More details about models and equations are reported in studies [6, 12, 21].

The further recovery of the engines’ exhaust gases waste heat for the electric and/or cooling power production, brings several energetic, environmental, and economic improvements, compared to the traditional systems of energy supply. The CHP-BESS solutions represent a starting point in the energy transition panorama for the healthcare buildings, as capable to reduce their environmental impact while allowing operations regardless of the power grid availability. However, to cover the forecasted energetic demand of the next years, many solutions for enhanced energy conversion efficiency are under investigation. The ORC and APC plants represent great solutions respectively for the electric and electric/cooling production, fueled by waste heat coming from upstream industrial processes or thermal power plants. Their integration inside a max PES-aimed optimized CHP-BESS solution has brought a PES and CO₂ reduction improvements, with a slight increase of the SPB. The characteristics of the APC plants are well suited when the cooling demand is consistent during the year, as its efficiency reaches the maximum value when converting thermal to cooling power despite to electric power; on the other side ORC plants should be preferred when the electric demand is dominant during the year, characterized by higher efficiency when converting thermal to electric power, compared to the one of the APC plants. However, the integration of these waste heat recovery plants inside an optimized CHP-BESS solution, altered the behavior and the outcomes of the system, which requires a new multi-input multi-variable optimization design process, in order to redefine the optimal layout capable to meet at the same time the maximization of the energetic and economic saving.