Use Of Low-grade Energy Research Articles

Water transportation using a low-height overhang structure along with an expansion pressure of bubbles in a nucleate boiling region

Effective use of low-grade energy is a vital issue for a more sustainable society. Here, we propose a water pump using a low-height overhang structure that effectively uses an expansion pressure of bubbles in a nucleate boiling region. Specifically, by fabricating a periodic overhang structure on a brass surface, we show that a ∼20 mm/s net flow can be generated in a nucleate boiling regime. In addition, by observing bubbles with a high-speed camera of 960 fps, we find that bubbles are generated selectively in the overhang structure and grow to push out water in a lateral direction. Furthermore, by proposing a simple model that considers the expansion pressure of bubbles, we succeed in explaining the experimental results to some extent. Our findings should contribute to the effective use of low-grade energy in a nucleate boiling regime.

Experimental and numerical study on thermal performances of a modular hydronic thermal barrier wall with heat injection system in filler cavities

The hydronic thermal barriers with dynamically adjustable thermal performance have become one of the highly anticipated solutions for opaque envelope insulation while improving the efficiency of the use of low-grade energy. In response to the technical constraints encountered by conventional hydronic thermal barrier walls (i.e., CHTBs) when it comes to enhancing heat transfer, the concept of modular hydronic thermal barrier walls (i.e., MHTBs) was introduced. The MHTB incorporates a heat injection system within its filler cavities, a feature designed to promote the overall thermal performance of the opaque enclosure structure. The performance enhancement described is mainly achieved through active management of the thermal conductivity of the filling material within the vicinity of fluid pipes. In this study, field thermal performance assessments were carried out during winter conditions in three reduced-scale experimental rooms. The southern boundary of these test rooms adopts walls with different configurations, i.e., MHTB, CHTB, and traditional thermal insulation wall. Subsequently, a comprehensive investigation regarding the impacts of filling materials on the energy performances of MHTB was conducted through numerical simulations. The outcomes demonstrated a substantial reduction in heat loss through the south wall by using MHTB, leading to reduced room thermal discomfort. In particular, with a charging temperature set at 35.0 °C, the inner surface temperature of MHTB displayed a noteworthy increase of 10.3 °C compared to that of the traditional thermal insulation wall, resulting in a significant reduction in heat load by a factor of 7.4. Additionally, it was observed that the filler material within the pipe cavity played a vital role in shaping MHTB's thermal performance. For instance, considering the case with a heat injection temperature of 25.0 °C, a doubling of thermal conductivity resulted in an enhancement of 0.24 W/m2 in heat gain on the outer surface and an overall increase of 0.04 MJ in total injected heat. This study demonstrated the positive effect of MHTB on optimizing the efficiency of hydronic thermal barriers and guided their further application and research.

Analysis on the thermal performance of low-temperature radiant floor coupled with intermittent stratum ventilation (LTR-ISV) for space heating

With increasing energy use and outbreaks of respiratory infectious diseases (such as COVID-19) in buildings, there is a growing interest in creating healthy and energy-efficient indoor environments. A novel heating system named low-temperature radiant floor coupled with intermittent stratum ventilation (LTR-ISV) is proposed in this study. Thermal performance, indoor air quality, energy and exergy performance were investigated and compared with conventional radiant floor heating (CRFH) and conventional radiant floor heating with mixing ventilation (CRFH + MV). The results indicated that LTR-ISV had a more uniform operative temperature distribution and overall thermal sensation, and air mixing was enhanced without generating additional draft sensation. Compared with CRFH and CRFH + MV, the indoor CO2 concentration in LTR-ISV can be reduced by 1355 ppm and 400 ppm, respectively. Airborne transmission risk can also be reduced by 5.35 times. The coefficient of performance for CRFH, CRFH + MV, and LTR-ISV during working hours was 4.2, 2.5, and 3.4, respectively. The lower value of LTR-ISV was due to the high energy usage of the primary air handing unit. In the non-working hours, LTR-ISV was 0.6 and 1.3 higher compared to CRFH and CRFH + MV, respectively. The exergy efficiency of LTR-ISV, CRFH, and CRFH + MV was 81.77 %, 76.43 %, and 64.71 %, respectively. Therefore, the LTR-ISV system can meet the requirements of high indoor air quality and thermal comfort and provides a reference for the energy-saving use of low-grade energy in space heating.

Development of thermally-driven hybrid LiBr-water absorption system for simultaneously supplying steam and refrigeration effect

• A hybrid LiBr-water absorption system for producing steam and refrigeration effect. • System characteristics with respect to part-load conditions. • The overall COP of the proposed hybrid absorption system of 0.463. • Experimental thermodynamic states concerning several operating modes. • Pay-back analysis of the proposed system compared to the existing business cases. Waste heat is a form of renewable energy discarded into the atmosphere. Waste heat-driven absorption systems (i.e., heat pumps and refrigeration systems) have attracted attention as district heating and cooling systems. This study proposes a waste heat-driven hybrid lithium bromide-water absorption system for the simultaneous generation of available steam and refrigeration effects. The proposed system is a form of the combined closed thermal cycle between the sub-heat pump and the sub-refrigeration system. It shares a generator and condenser, which allows the system to be more compact and flexible. We first present a description of the proposed system, designed for a cooling capacity of 350 kW with a steam generation rate of 200 kg/h. Our preliminary study proposed a pilot-scale system to clarify the flexible and simultaneous control means of the heating and cooling capacities by partially distributing the flow rate of the LiBr solution entering each sub-cycle. In addition to the results from the preliminary study, the present work examined the effect of part-load conditions on the operating parameters of the hybrid absorption system. The results showed that the coefficient of performances of sub-refrigeration and sub-heat pump cycle were approximately 0.787 and 0.658, respectively, and as a result, the total coefficient of performance of 0.463 was achieved. The theoretical model was validated by the experimental data, and the payback analysis was suggested to assure the economic validity of the proposed system. It was noted that the proposed hybrid absorption system demanded high initial facility cost than the comparison groups, but a payback of 2.79 and 2.36 years was evaluated because of the use of low-grade energy.

Thermodynamic modelling and parametric study of a low temperature vapour compression-absorption system based on modified Gouy-Stodola equation

Present paper thermodynamically analyses a VCAS (vapour compression-absorption system) with carbon dioxide (compression section) and ammonia-water (absorption section) as refrigerants and determines the optimal condensing temperature of cascade condenser using modified Gouy-Stodola equation. The optimum cascade condenser temperature is found to be −13 °C for 175 kW refrigeration capacity at an evaporator temperature of −45 °C and condenser temperature of 35 °C. The optimum cascade condenser temperature maximises the overall COP, rational efficiency and minimises the total irreversibility rate of the VCAS system. The value of optimum condensing temperature and its corresponding maximum COP, and minimum irreversibility rate are discussed for a wide range of operating conditions. Further, a comparative study of TSVCS (two stage vapour compression system) used for low temperature refrigeration applications with VCAS shows that at design point, primary energy consumption is reduced by 60.6% and electrical COP is improved by 153.6% in VCAS as compared to conventional TSVCS. But the total irreversibility rate of VCAS is 38.4% higher than the TSVCS due to the use of low grade energy in vapour absorption system and hence the rational efficiency of VCAS is 14% low.

A new power generation method utilizing a low grade heat source

Energy crisis make the effective use of low grade energy more and more urgent. It is still a worldwide difficult conundrum. To efficiently recover low grade heat, this paper deals with a theoretical analysis of a new power generation method driven by a low grade heat source. When the temperature of the low grade heat source exceeds the saturated temperature, it can heat the liquid into steam. If the steam is sealed and cooled in a container, it will lead to a negative pressure condition. The proposed power generation method utilizes the negative pressure condition in the sealed container, called as a condensator. When the condensator is connected to a liquid pool, the liquid will be pumped into it by the negative pressure condition. After the condensator is filled by liquid, the liquid flows back into the pool and drives the turbine to generate electricity. According to our analysis, for water, the head pressure of water pumped into the condensator could reach 9.5 m when the temperature of water in the pool is 25 °C, and the steam temperature is 105 °C. Theoretical thermal efficiency of this power generation system could reach 3.2% to 5.8% varying with the altitude of the condensator to the water level, ignoring steam leakage loss.

Development of Advanced power generation cycle applied adsorption process by use of low-grade Energy

Development of Advanced power generation cycle applied adsorption process by use of low-grade Energy

The use of a low-grade thermal energy in a combined refrigeration system for multi-temperature cold production

No Abstract Available Journal of the Cameroon Academy of Sciences Vol.4(3) 2004: 247-254