Two-tank Systems Research Articles

Influence of flow distribution on the thermal performance of dual-media thermocline energy storage systems

Dual-media molten-salt thermocline thermal energy storage (TES) systems can be used to maintain constant power production at Concentrated Solar Power (CSP) plants independent of weather changes at costs less than that of traditional two-tank molten-salt storage systems. The flow distribution is a critical parameter affecting the thermal performance but has rarely been considered for dual-media TES systems in previous studies. This study analyzes the influence of the flow distributions at the inlet and outlet of a salt-rock dual-media thermocline TES tank on the thermal performance. The flow distribution is characterized by radial component, and a two-temperature model is used to investigate the thermal performance of the thermocline tank. The model is first validated against experiment data available in the literature and then used to study the discharge process of the thermocline thermal storage tank for various flow distributions. The results show that even with a large (80% of the area) flow blockage at the inlet, the flow distribution has only a limited influence on the useable energy output (<3% change) of the dual-media storage tank. In fact, the flow non-uniformities reduce the thickness of the thermocline layer and slightly increase the useable energy output, whereas non-uniformities at the top outlet slightly decrease the output. An entropy generation analysis, including the effects from diffusion and interstitial heat transfer, is performed to further explain these phenomena. The interstitial heat transfer is found to be the main cause for the entropy generation in the discharge. Flow non-uniformities are also found to reduce the entropy generation.

A cost and performance comparison of packed bed and structured thermocline thermal energy storage systems

A structured concrete thermocline thermal energy storage (TES) system is proposed as an alternative to currently-used TES systems. The issues of material settlement and thermal ratcheting found in packed bed thermocline TES systems is avoided by replacing the packed aggregate bed with structured high-temperature concrete. A summary of all utility scale TES systems with integrated TES in existence today is provided and discussed. Cost reduction options such as replacing two-tank systems with single-tank systems and replacing liquid storage media with solid storage media are discussed along with limitations of both options. Numeric models are developed to simulate the performance of utility scale packed bed and structured thermocline TES systems; efficiencies of 92.37% and 84% are modeled for packed-bed and structured systems. A complete cost analysis of utility-scale, 2165MWh packed bed and structured systems is conducted; capacity costs of $30/kWh and $34/kWh are determined for packed bed and structured systems respectively. A structured concrete thermocline is deemed to be a viable TES option due to its low cost and the fact that there are no concerns of thermal ratcheting of the tank.

Spatial and temporal modeling of sub- and supercritical thermal energy storage

This paper describes a thermodynamic model that simulates the discharge cycle of a single-tank thermal energy storage (TES) system that can operate from the two-phase (liquid–vapor) to supercritical regimes for storage fluid temperatures typical of concentrating solar power plants. State-of-the-art TES design utilizes a two-tank system with molten nitrate salts; one major problem is the high capital cost of the salts (International Renewable Energy Agency, 2012). The alternate approach explored here opens up the use of low-cost fluids by considering operation at higher pressures associated with the two-phase and supercritical regimes.The main challenge to such a system is its high pressures and temperatures which necessitate a relatively high-cost containment vessel that represents a large fraction of the system capital cost. To mitigate this cost, the proposed design utilizes a single-tank TES system, effectively halving the required wall material. A single-tank approach also significantly reduces the complexity of the system in comparison to the two-tank systems, which require expensive pumps and external heat exchangers.A thermodynamic model is used to evaluate system performance; in particular it predicts the volume of tank wall material needed to encapsulate the storage fluid. The transient temperature of the tank is observed to remain hottest at the storage tank exit, which is beneficial to system operation. It is also shown that there is an optimum storage fluid loading that generates a given turbine energy output while minimizing the required tank wall material.Overall, this study explores opportunities to further improve current solar thermal technologies. The proposed single-tank system shows promise for decreasing the cost of thermal energy storage.

High oxygen consumption rates and scale loss indicate elevated aggressive behaviour at low rearing density, while elevated brain serotonergic activity suggests chronic stress at high rearing densities in farmed rainbow trout

The effect of stocking density on indicators of welfare has been investigated by several studies on farmed rainbow trout Oncorhynchus mykiss. However, the densities at which welfare are compromised remain ambiguous. Here three different stocking density treatments were selected based on the results of a previous study, where levels of crowding where determined using the spatial distribution of fish in two-tank systems. An un-crowded low density of 25kgm−3, the highest density accepted by the fish without showing indications of crowding stress of 80kgm−3 as the intermediate density, and the highest density accepted by the fish showing indications of crowding stress of 140kgm−3 as the high density were investigated. The aim of the present study was to examine the effect of being held at these densities on indicators of welfare. This was achieved through oxygen consumption measurements using automated respirometry, recording fin erosion, determining scale loss and analysing plasma cortisol and brain serotonergic activity levels. The results obtained in the present study indicated that at the lowest density the fish had the space and opportunity to display their natural aggressive behaviour and that the fish held at the highest density were exposed to a situation of confinement.

Utilising spatial distribution in two-tank systems to investigate the level of aversiveness to crowding in farmed rainbow trout Oncorhynchus mykiss

In aquaculture, fish are exposed to a range of unfavourable environmental conditions. Amongst these, stocking density has attracted considerable attention as inappropriate densities may compromise welfare and negatively impact production. However, the recommendations for stocking remain elusive. The aim of the present study was to apply a novel method to investigate a level of crowding that indicated aversiveness in rainbow trout (Oncorhynchus mykiss). In a two-tank system, where two identical tanks were connected via a doorway, it was observed that social behaviour controlled the distribution of the fish between the tanks. Fish were stocked at equal quantities in each tank of the system. The doorway was opened and the fish moved between the two tanks. Typically, this resulted in one tank being occupied by a few highly aggressive dominant individuals (“dominant” tank) and the majority of the fish occupying the second tank (“crowded” tank). Here, the potential of this unequal spatial distribution for quantifying aversion to crowding was explored. Fish were stocked in three two-tank systems at a total density of 20, 40 and 80kgm−3 respectively. The number of fish in each tank was determined every three days throughout the duration of the experiment and the percentage of fish in the “crowded” tank was used as an indicator of the distribution pattern in the two-tank systems. The results indicated a negative relationship between the total density stocked (20, 40 and 80kgm−3) and the percentage of fish in the “crowded” tank. A subsample of individuals was sacrificed for blood and brain samples every three days from the “crowded” tank, prior to the fish count. The neuroendocrine indicators of stress, elevated serotonergic activity levels which were not associated with high plasma levels of cortisol, suggested chronic stress in the fish at the highest total density stocked (80kgm−3). Taken together, these results indicated that a level of aversiveness to crowding had been reached at the highest density stocked, where the mean absolute density, irrespective of time of day, observed in the “crowded” tank was 126.5±3.7kgm−3.

An improved design method for solar water heating systems

In this paper, the Φ ,f-chart method is extended to open-loop solar hot water heating systems. This method is an improvement over the f-chart method since it does not impose any restrictions on the water set temperature, water mains temperature, or the preheat tank loss coefficient. The procedure is general for both one-tank and two-tank systems. The modified Φ ,f-chart method is still applicable to the closed-loop systems for which it was originally developed.

Measurement of rock pile heat transfer coefficients

Several authors have established single-tank packed-bed storage as a promising alternative that can be coupled with renewable thermal energy sources. The use of such systems can ensure a cost reduction of approximately 33%, compared to two-tank systems, which represents the dominating solution for high-temperature storage. Herein, an overview of the modeling approaches for assessing the yield and efficiency of packed-bed energy storage systems is presented. Additionally, the validation approaches used as well as the conventional materials and structures employed in such systems are described. One of the critical issues that affect the simulation models’ performance in packed-bed energy storage systems is the treatment of the radiation exchange among particles. The impacts of the radiation phenomena on the overall performance and internal temperature distribution of the tank, which facilitate the identification of the operating conditions within the influence of radiation and are significant for analytical purposes are discussed herein. Through parametric analysis, it is demonstrated that the radiation heat transfer coefficient could be as high as 32% of the magnitude of the convection coefficient; thus, it should not be underestimated when analyzing operation temperatures above 750 °C.