Comparative analysis of thermal storage options for industrial steam generation in a solar thermal integrated system

Project background

Heat is half of the global primary energy consumption, and results in nearly 40 % of the Global CO2 emissions. Industries consume a lot of heating at medium temperature range as well. There are several technologies which can produce heating for industries with low carbon emissions. These technology incudes solar thermal (ST) collectors, high temperature heat pump (HTHP), and boilers using green fuels such as waste biomass. The cost competitiveness of these technologies is important to analyses design a cost-effective heating solution for industries. Solar thermal collectors such as the one produced by Absolicon solar can be used for industries to generate steam. These collector, similar to any other solar thermal technology, generates steam at suitable temperature, and can serve industrial use. However, to reach a high solar fraction (fraction of heat provided by solar field in comparison to total heating demand), a thermal storage is imperative. Steam is impractical to store due to the very low density, and thus typical method is a pressurised storage tank, which store heat in form of water, which is further converted to steam. As the cost of tank storage is linear to tank volume m³, the higher solar fraction often tends to underutilize the storage due to mismatch in energy available and load demand. So sizing the tank storage for peak is not cost optimal at higher solar fraction, as it increases the levelized cost of heating (LCOH) non-linearly. If we size the tank for peak, the energy storage density keeps decreasing, while the cost keeps increasing linearly, resulting in increasing LCOH. One method to avoid this effect is to use a storage with cheap materials so the energy density can be increasing without linear increase in the cost. Analysis of such storage in form of rock-bed storage, and its comparison to classical pressurised storage is the focus of this study.

Aim of the project

The aim is to compare the techno-economic performance for a solar thermal integration when used with 2 different types of storage systems: 1) A typical pressurized tank storage; 2)Novel stone storage by DTU based on stones. The focus will be to compare the LCOH of solar + storage system at fixed Solar fractions (For e.g 10 % to 80 % SF, in steps of 10 % for e.g) for a given industrial load profile. Due to the cost effectiveness of stone storage, after a certain storage size limit, the stone storage could be better to integrate than pressurized storage, due to the non-linear storage cost trend in former case.

The focused storage is developed by DTU, which is a partially underground rock storage using Swedish Diabase and a vertical air flow. The storage housing used reinforced concrete that was applied by spraying at the construction site. A 5 mm stainless steel shell was placed between the rock and the insulation material to prevent the rocks from damaging its housing material from the mechanical stress caused by heating and cooling the rock bed.

Tools and methods

The thesis would require expertise in different areas which are provided by the various supervisors. The industrial boundary conditions and simulation of solar thermal collectors, along with data on classical pressurised storage will be facilitated by Absolicon.
The model on simulation of novel rock bed storage will be along with the cost functions will be provided by the partners in collaboration with DTU. The model is written in Python, and the cost calculations can be performed in Excel.
KTH will support the student on the optimisation of overall system and case studies identification.
More refined boundary conditions can be defined with supervisors.

Main Deliverables

The main deliverables of the project include:

Final project report and presentation comprising description of project, literature review on current TES technologies and solar thermal systems integration, TES solutions comparison and benchmarking with other commercial alternatives, and final suggestions.
Techno-economic models: models and user guidelines / instructions.