Integrated Energy Flexibility Management at Wastewater Treatment Facilities.

Energy flexibility quantification of grid-responsive buildings: Energy flexibility index and assessment of their effectiveness for applications

Flexibility categorization, sources, capabilities and technologies for energy-flexible and grid-responsive buildings: State-of-the-art and future perspective

Chapter 2 - Building energy flexibility: definitions, sources, indicators, and quantification methods

Sewage treatment plants (STPs) are evolving from pollutant removal facilities to resource and energy recovery factories. In the current sewage treatment process, majority of the organic carbon is oxidized to CO2, which wastes the energy embedded within. To enhance energy recovery from sewage, a novel sewage treatment design has been proposed, which is attracting wide attention internationally. It involves an upfront separation of organic carbon from sewage. The separated carbon can then be converted to biogas via anaerobic digestion. Compared to the conventional process, this newly proposed process could substantially enhance energy recovery from sewage. While the water industry is keen to adopt the new design, the implementation is stilled confronted with two key challenges, regarding the wastewater and sludge streams, respectively.1) For the wastewater stream, satisfactory nitrogen removal is the main challenge. Considering majority of the organic carbon in sewage has been redirected for bioenergy recovery, the nitrogen in sewage cannot be removed via the conventional nitrification and denitrification process, but could be removed via the partial nitritation-anammox (PN/A) process, which does not require organic carbon. However, the implementation of PN/A in sewage treatment is hindered by the difficulty in stable suppression of nitrite-oxidizing-bacteria (NOB). NOB would convert nitrite to nitrate, which cannot be removed through the anammox process.2) Concerning the sludge stream, the main issue is to reduce sludge disposal cost, which accounts for up to 60% of the total costs in STPs. Anaerobic digestion process can only achieve around 30% solids reduction. Besides, the anaerobically-digested (AD) sludge is usually restricted from wider agricultural reuse due to the fact that the pathogen and toxic metals levels often exceed the thresholds for Class A biosolids.To address the first challenge, two NOB suppression strategies were adopted in this thesis (Chapter 5), namely the sidestream sludge treatment with free ammonia (FA) to inactivate NOB, and the use of low dissolved oxygen (DO) to suppress NOB growth. In the experimental study over a period of 550 days, these two strategies were stepwise applied. However, it was found that these strategies, although effective initially, lost efficacy over an extended period of time due to NOB adaptation. FA sludge treatment alone sustained nitrite accumulation for about two months, after which NOB adaptation occurred causing PN to fail. Low DO at 0.2–0.4 mg O2 L-1 was then applied, in conjunction with FA treatment, which successfully re-established PN. However, new and unidentified NOB with a higher apparent affinity with oxygen emerged in three months, again leading to PN failure. In Chapter 6, in-situ nitrite-scavenging was introduced as the third strategy, by adding anammox carriers to the sludge, thus forming an Integrated Film and Activated sludge (IFAS) system. The combo-strategy, consisting of FA-based sidestream sludge treatment, low DO, and nitrite-scavenging, delivered reliable NOB suppression with no further adaptation in the remaining experimental period (eight months). The resulted one-stage PN/A reactor achieved a nitrogen removal efficiency of 84.2 ± 5.4%. In parallel to in-situ nitrite-scavenging, a different approach to tackling NOB adaptation, i.e. in-situ FNA inhibition, was also investigated (Chapter 7). This is achieved through acidic nitritation operated at pH 4.5–5.0, in which any nitrite accumulation would form considerable FNA in-situ to suppress NOB. Nitrite accumulation with a high NO2-/(NO2- + NO3-) ratio of 95 ± 5% was stably maintained for more than 300 days. A volumetric NH4+ removal rate of 188 ± 14 mg N L-1 d-1 was obtained, which is comparable to the ammonium oxidation rate achievable in a conventional nitrogen removal process. 16S rRNA gene sequencing analyses revealed the dominance of a new AOB genus, ‘Ca. Nitrosoglobus’, in the nitrifying guild. This is the first identification of Ca. Nitrosoglobus as key ammonia-oxidizers in a wastewater treatment system. It was found that Ca. Nitrosoglobus can tolerate low pH and FNA at levels that completely inhibit/inactivate other AOB and NOB commonly found in wastewater treatment processes.The strong FNA resistance of Ca. Nitrosoglobus was further employed to achieve deep reduction and stabilization of AD sludge in an acidic post-anaerobic aerobic digestion (PAAD) process (Chapter 8). The acidic condition was self-driven by Ca. Nitrosoglobus, which produced protons by oxidizing the ammonium in AD sludge. Through inoculating Ca.Nitrosoglobus into the PAAD, the pH dropped from 7.9 ± 0.2 in the feed sludge to 4.8 ± 0.2 in the PAAD reactor. At the acidic pH, nitrite accumulated at 200.0 ± 17.2 mg N L-1, from which an in-situ FNA of 8.5 ± 1.8 mg HNO2-N L-1 formed. As a combined result of low pH and high FNA, the total solids in AD sludge were substantially reduced by 25.2 ± 7.0%, in contrast to 8.3 ± 3.1% in the neutral-pH PAAD reactor operated in parallel. Additionally, the acidic PAAD process improved the stabilization level of AD sludge from Class B to Class A standards in terms of pathogen levels and specific oxygen uptake rates.Based on the acid-tolerance property of Ca. Nitrosoglobus, a novel bioleaching approach was developed to remove the toxic metals in AD sludge (Chapter 9). This bioleaching approach is based on microbial oxidation of the innate ammonium in AD sludge at a leaching-inducing pH (2–3). A highly enriched Ca. Nitrosoglobus consortium (72.5 ± 2.3% based on 16S rRNA gene sequencing) was enriched after 120 days in a laboratory sequencing batch reactor. It was found that the consortium maintained ammonium oxidation even at pH 2.5, with approximately 30% of its maximum activity (measured at the optimal pH of 5.5). Through inoculating the consortium into AD sludge, pH of the AD sludge decreased from 7.5 to 2.0 over a five-day aeration period, driven by the protons generated from ammonium oxidation. As a result, the metals in AD sludge were efficiently extracted into the liquid phase. Particularly, two of the most abundant toxic metals Cu and Zn were solubilized with high efficiencies of 88 ± 4% and 96 ± 3%, respectively. These results demonstrated the feasibility of metal removal from biosolids without external chemical addition, enabling an economical disposal and safe reuse of the excess sludge generated during biological sewage treatment. The three acidic activated-sludge processes above all hinge on a pivotal microorganism, Ca. Nitrosoglobus. To facilitate the technological development of the acidic activated-sludge processes, Ca. Nitrosoglobus was characterized stoichiometrically and kinetically (Chapter 10). The yield, the maximum growth rate, and the decay rate were estimated. The results indicate a relatively low net growth rate compared to other AOB species. Ca. Nitrosoglobus also have relatively low affinities to oxygen and total ammonia, compared to other AOB commonly found in activated sludge. These render Ca. Nitrosoglobus uncompetitive with traditional r-strategist or K-strategist ammonia-oxidizers in current activated-sludge systems. However, Ca. Nitrosoglobus possesses strong resistance to acidic pH (as low as 2.0) and high FNA (IC50 of 29.5 mg HNO2-N L-1). The acid- and FNA-resistance made Ca. Nitrosoglobus an A-strategist prospering in acidic activated-sludge systems, where no other ammonia oxidizers or nitrite oxidizers can survive.In summary, revolving a new acid-tolerant AOB Ca. Nitrosoglobus, this thesis develops three innovative technologies, which expand the operational pH of activated-sludge process from near neutral (~7) to acidic range (2–6). The established acidic activated-sludge processes have opened up new opportunities to address the main challenges confronted by STPs, i.e. attaining satisfactory pollutant removal while minimizing operational cost, which would promote the on-going paradigm shift in STPs from pollutant removal to resource and energy recovery.

Existing grids have been designed with traditional large centralized generation in mind; however, with the ever-increasing utilization of renewable distributed energy resources, the challenges of proper grid management have intensified. Demand-side energy flexibility is seen as one potential way to alleviate these challenges. Presently, residential demand-side energy flexibility has remained a largely untapped resource since individual prosumers are too small to provide enough capacity, thus necessitating the need for an aggregator. In view of the aforementioned, this paper conducts a literature review on the aggregated residential demand-side energy flexibility. The paper gives an overview of characterization methods of energy flexibility. The sources of residential energy flexibility are identified and categorized based on their flexibility characteristics. In addition, the quantification methods and parameters of energy flexibility are analyzed. Moreover, the forecasting methods of energy flexibility in the context of different flexibility sources are outlined. Additionally, an overview of existing markets and potential new emerging flexibility markets is given. The challenges and barriers faced by the aggregators attempting to enter flexibility markets are examined. Finally, the paper is concluded by providing a discussion of the key findings that summarize the current research directions and highlight the gaps for future development of aggregated energy flexibility.

Electric vehicles (EVs) can play an ancillary role in energy and electricity system management. Vehicle-to-Grid (V2G) technology allows EV batteries to be discharged back into the grid. This technology enables charging when electricity prices are low and there is abundance of electricity in the grid and discharging when electricity costs are high and there is high load in the grid. The current study estimates the combination of financial compensation (FC) incentives (reduction in monthly electricity bill) and minimum guaranteed charge (MC) that would be needed to increase the acceptance of V2G technology among Norwegians. Estimating a multi-equation econometric model, we investigate how socioeconomic, geographical, and psychological exogenous variables predict the level of FC and required MC, as well as the relationship between FC and MC. We found that there is a mutual and negative relationship between FC and MC. Based on the MC-FC economic relationship, the V2G system is more likely to be accepted by older people, people who perceive the V2G system as more useful, people who have EV experience, and individuals with a higher level of trust in the V2G system. The group with strong trust in V2G demands less FC for a given value of MC. When MC is reduced, younger age groups (18–22) are more likely to demand higher FC. Our estimations also show that people demand an average reduction in electricity bills of 144 USD (72% of the average monthly electricity bill) as compensation for V2G investment while they would also use V2G if their electric car’s battery had a minimum level of 71% power. Monetary incentives based on socioeconomic status, options in the interface allowing the user to easily override the standards, and trusted methods of calculating the revenue may be considered in order to reduce financial expectations and concerns regarding minimum battery charges.

This paper proposes a real time price based optimal scheduling mechanism for centralized air conditioning load. In residential context, air conditioners are the main reason for peak load in summers. The proposed mechanism aims to determine optimal operation of air conditioner such that it results in reduction in consumer's electricity bills. Mixed Integer Programming technique is used for obtaining optimal schedule. The results obtained are compared with a previously established Linear Sequential Algorithm for evaluating the effectiveness of the scheme. The results show that Mixed Integer Programming technique provides higher reduction in electricity bill as compared to Linear Sequential technique.

Retail buildings has an important role for demand side energy flexibility because of their high energy consumption, variety of energy flexibility resources, and centralized control via building control systems. Energy flexibility requires agreements and collaborations among different actors. However, the stakeholders' reaction to energy flexibility have not been fully investigated. Therefore, this paper aims to investigate the stakeholder involvement in energy flexibility by applying the business ecosystem concept (including actors, relationships, value alliances, and influential factors), with the discussion of the stakeholders' roles and their interrelation in delivering energy flexibility with the influential factors to the actual implementation of energy flexible operation of their buildings. Based on a literature analysis, the results cover stakeholders' types and roles, perceptions (drivers, barriers, and benefits), energy management activities and technology adoptions, and the stakeholders' interaction for the energy flexibility in retail buildings.

Towards greener production, manufacturing companies face several challenges, for example peak load shaving or flexible production planning as parts of demand-side management (DSM). DSM uses processes that can be shut down, shifted, or controlled. Advances in digitalization in the energy sector and manufacturing systems create transparency which in turn offers new opportunities to commercialize energy flexibility potentials as optimally and automatically as possible. The variety of flexibilities in manufacturing systems and various dependencies of different kinds of complex manufacturing processes complicate the modelling and aggregation of flexibility. To overcome this challenge, we developed a method for the aggregation of energy flexibilities that is based on a generic energy flexibility data model. The method proposes a two-step approach to aggregate flexibilities cost efficiently and considers manufacturing specific limitations. For cost-efficient aggregation, we use in the first step the merit-order model known from the energy industry and in the second step the bin-packing problem originating from combinatorial optimization, adapted according to the generic data model. The two-step approach allows energy flexibilities to be aggregated across industries, facilities, and systems, thus ensuring broad applicability.KeywordsDemand-side managementEnergy flexible manufacturingEnergy flexibility aggregation

A focal point of ongoing research is matching the energy demand in the built environment to the energy supply from onsite generation, to maximize the self-consumption, and from the energy grids, to lower energy costs and reduce peak loads on the system. Energy flexibility addresses this task by modulating the energy demand in a building according to dynamic criteria such as electricity prices or onsite generation. This study addresses the potential of building performance simulation with real time rule-based control that provides energy flexibility based on onsite generation and hourly electricity prices, prioritizing energy matching, and reducing costs. The novelty relies on investigating four sources of energy flexibility simultaneously: shiftable machine loads, charging/discharging of batteries, hot-water storage tanks, and the building’s mass. The energy matching and flexibility actions provided a decrease of up to 4% in annual energy costs, yet risk increasing the cost by 9% when the savings are offset by the increase in the energy demand. As well, the method for price categorization strongly influences the cost performance of the flexibility actions. The outcomes of this study provide insight to energy flexibility sources in nearly-zero energy buildings and how their outcomes are affected by price thresholds.

Characterizing energy flexibility of buildings with electric vehicles and shiftable appliances on single building level and aggregated level

Intermittent demand-controlled ventilation for energy flexibility and indoor air quality

Energy flexibility and viability enhancement for an ocean-energy-supported zero-emission office building with respect to both existing and advanced utility business models with dynamic responsive incentives

A short bibliographic review concerning biomethane production from wastewater sludge.

The impacts of engineered nanomaterials (ENMs) on anaerobic digestion processes