• Energy Research
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In this work, a novel enhanced deep borehole heat exchanger (EDBHE) was proposed to improve heat extraction efficiency based on the jet grouting method. By means of this technology, a soilcrete zone with high thermal conductivity was built near the wellbore. To analyze the feasibility and efficiency of this method, we firstly constructed a validated deep borehole heat exchanger (DBHE) model based on the field experimental data. Numerical simulations were carried out to investigate the 30-year production performance of EDBHE. Results demonstrated that the jet grouting method is an efficient way for improving thermal output of DBHE. It is evaluated that the average annual heat production rate over a 30-year heating period of EDBHE is 463.2 kW, which is 1.27 times as that of DBHE. Sensitivity analyses indicate that the heat production rate and outlet temperature mainly depend on the height and radius of the artificial soilcrete zone. However, thermal output is not sensitive to thermal conductivity of the soilcrete zone due to the higher thermal resistance of the geological formation. For the experimental site used in this work, the recommended height, radius, and thermal conductivity of the soilcrete are 1000 m, 1.0 m, and 50 W/m °C, respectively.

Abstract Field experiences have shown that sub-synchronous oscillation (SSO) can occur in direct-drive wind farms with VSC-HVDC systems. Due to the complexity of the detailed wind farm model, a dynamic equivalent model, with a reasonable order reduction of the detailed model and still reflecting inside-wind-farm and wind-farm-grid SSO characteristics is essential. In this paper, based on the principle that similar matrices have identical eigenvalues, the SSO analysis of an N-machine wind farm with VSC-HVDC system is conducted by simplifying it into two single-machine systems. The modeling method of the two single-machine systems is presented. Four case studies are presented to verify the effectiveness of the proposed model when compared with the detailed model in various scenarios. The proposed model is also benchmarked with the output multiplication-based equivalent model (OMM). Comparison results show that although the system order is reduced significantly, the proposed simplified equivalent model can still reflect inside-wind-farm and wind-farm-grid SSO modes in various scenarios. Meanwhile, the rationality of the OMM in terms of the wind-farm-grid SSO analysis is verified theoretically.

Abstract With the large-scale integration of the distribution generations (DGs) and the increasing medium-voltage and low-voltage DC power demands, multi-terminal hybrid AC/DC microgrid has drawn great attention from researchers around the world. In order to reduce the number of power conversion stages and meet DC transmission demands under different DC voltage levels, this paper proposes a four-terminal interconnection scheme of the hybrid AC/DC microgrid, connecting one medium-voltage AC (MVAC) terminal, one medium-voltage DC (MVDC) terminal and two low-voltage DC (LVDC) terminals. The proposed interconnection scheme includes a modular multilevel converter (MMC) as the main interlinking converter of the MVAC grid and MVDC microgrid, and a series of dual active bridges (DAB) converters as two isolated LV DC microgrid interfaces. It has more flexibility for power supplies, especially MVDC transmission, and a more robust tolerance for unequal power distribution between the two LVDC Microgrids. To realize the DC capacitor voltage balancing control, an improved energy control method is proposed in this paper. The proposed method keeps DC capacitor voltage balance and AC current zero on the MVDC transmission lines, which contributes to the stability of the MVDC microgrid. In addition, the symmetry of the AC currents is also guaranteed with this control method. Validation results of a four-terminal hybrid AC/DC microgrid verify the effectiveness of the proposed microgrid and control scheme.

Fuel moisture limits the availability of fuel to wildfires in many forest areas worldwide, but the effects of climate change on moisture constraints remain largely unknown. Here we addressed how climate affects fuel moisture in pine stands from Catalonia, NE Spain, and the potential effects of increasing climate aridity on burned area in the Pyrenees, a mesic mountainous area where fire is currently rare. We first quantified variation in fuel moisture in six sites distributed across an altitudinal gradient where the long-term mean annual temperature and precipitation vary by 6-15 °C and 395-933 mm, respectively. We observed significant spatial variation in live (78-162%) and dead (10-15%) fuel moisture across sites. The pattern of variation was negatively linked (r = |0.6|-|0.9|) to increases in vapor pressure deficit (VPD) and in the Aridity Index. Using seasonal fire records over 2006-2020, we observed that summer burned area in the Mediterranean forests of Northeast Spain and Southern France was strongly dependent on VPD (r = 0.93), the major driver (and predictor) of dead fuel moisture content (DFMC) at our sites. Based on the difference between VPD thresholds associated with large wildfire seasons in the Mediterranean (3.6 kPa) and the maximum VPD observed in surrounding Pyrenean mountains (3.1 kPa), we quantified the "safety margin" for Pyrenean forests (difference between actual VPD and that associated with large wildfires) at 0.5 kPa. The effects of live fuel moisture content (LFMC) on burned area were not significant under current conditions, a situation that may change with projected increases in climate aridity. Overall, our results indicate that DFMC in currently fire-free areas in Europe, like the Pyrenees, with vast amounts of fuel in many forest stands, may reach critical dryness thresholds beyond the safety margin and experience large wildfires after only mild increases in VPD, although LFMC can modulate the response.

The most extreme environments are the most vulnerable to transformation under a rapidly changing climate. These ecosystems harbor some of the most specialized species, which will likely suffer the highest extinction rates. We document the steepest temperature increase (2010-2021) on record at altitudes of above 4,000 m, triggering a decline of the relictual and highly adapted moss Takakia lepidozioides. Its de-novo-sequenced genome with 27,467 protein-coding genes includes distinct adaptations to abiotic stresses and comprises the largest number of fast-evolving genes under positive selection. The uplift of the study site in the last 65 million years has resulted in life-threatening UV-B radiation and drastically reduced temperatures, and we detected several of the molecular adaptations of Takakia to these environmental changes. Surprisingly, specific morphological features likely occurred earlier than 165 mya in much warmer environments. Following nearly 400 million years of evolution and resilience, this species is now facing extinction.

Abstract With the increasing access of renewable energy resources and fast ubiquitous connection of everything, the traditional one-way power flow from centralized generation to end consumers will give way to bidirectional-way power flow with multidirectional energy network among central grids and distributed prosumers. To empower the prosumer-centric Energy Internet (EI) and enhance the integration of energy-aware services, digitalization and decentralization are the key enablers to achieve transactive EI. This article presents a systematic overview on how Internet of Things (IoT) drives the digitalization of transactive EI and how blockchain empowers the decentralization of transactive EI. A comprehensive discussion on the key infrastructures is provided for presenting how to implement digitalization and decentralization of transactive EI, including the last mile “Advanced metering infrastructure” (AMI), renewables integrator “smart inverter”, energy flow adjuster “energy router”, and coordinator “Microgrid”. Challenges and future trends are discussed from an extensive point of view, including energy physical space, data cyber space and human social space.

AbstractThe recent rise in atmospheric methane (CH4) concentrations accelerates climate change and offsets mitigation efforts. Although wetlands are the largest natural CH4 source, estimates of global wetland CH4 emissions vary widely among approaches taken by bottom‐up (BU) process‐based biogeochemical models and top‐down (TD) atmospheric inversion methods. Here, we integrate in situ measurements, multi‐model ensembles, and a machine learning upscaling product into the International Land Model Benchmarking system to examine the relationship between wetland CH4 emission estimates and model performance. We find that using better‐performing models identified by observational constraints reduces the spread of wetland CH4 emission estimates by 62% and 39% for BU‐ and TD‐based approaches, respectively. However, global BU and TD CH4 emission estimate discrepancies increased by about 15% (from 31 to 36 TgCH4 year−1) when the top 20% models were used, although we consider this result moderately uncertain given the unevenly distributed global observations. Our analyses demonstrate that model performance ranking is subject to benchmark selection due to large inter‐site variability, highlighting the importance of expanding coverage of benchmark sites to diverse environmental conditions. We encourage future development of wetland CH4 models to move beyond static benchmarking and focus on evaluating site‐specific and ecosystem‐specific variabilities inferred from observations.

Phosphate tungsten bronze (WPB) and phosphate molybdenum bronze (MoPB) were synthesized and modified with rhenium. The existing phases were established by X-ray powder diffraction (XRPD), electron paramagnetic spectroscopy (EPR) and Field emission scanning electron microscopy (FESEM). The electroactivity of bronze samples, with and without rhenium for hydrogen evolution reaction (HER) was tested. The influence of carbon black presence in the catalytic ink on the electrochemical activity was investigated. Collected results provide insight into the effects of the constituents of an electrode on its electrochemical activity.