# Feedback Characterization for Enhanced Heat Recovery from Composing Processes: A Review
## 1 Introduction  
Compost Heat Recovery Systems (CHRSs) represent an eco-friendly approach aimed at capturing the heat generated during the composting of organic waste. This waste management technique supports global sustainability goals by reducing greenhouse gas (GHG) emissions while also reducing dependency on fossil fuels and optimizing resource usage. As organic matter breaks down in composting by microbes, heat is released, and this heat can be effectively captured and utilized for various purposes, such as heating spaces, warming water, and generating electricity when managed efficiently in compost systems. A research project explored a thermoelectric heat recovery system with 120 thermoelectric generators (TEGs), producing 11.3 volts and powering an LED strip while charging supercapacitors and operating sensors over 33 days [1], demonstrating the potential for electricity generation from compost heat. The ranges of potential thermal power output from CHRSs were reviewed by [PERSON] et al., who reported values ranging from 0.02 to 0.23 kW/m\({}^{3}\) depending on plant size, biomass type, and biodegradation duration [2]. For real-scale systems (\(\geq\)30 m\({}^{3}\)) operating over one year, the output ranges from 0.05 to 0.1 kW/m\({}^{3}\). For instance, a 55 m\({}^{3}\) plant can generate 2.75-5.5 thermal kW. An average plant using woodchips, running 12 h daily at 0.1 kW/m\({}^{3}\), could produce approximately 24,000 kWh of thermal energy annually. However, these values are provided without accounting for the heat recoveryefficiency of the current design of CHRS, a critical yet underexplored parameter that needs further investigation. CHRSs can be seen as an important process in facilitating the European Union's sustainability aspirations based on the EU Directive 2018/2001 [3] along with the upcoming Common Agricultural Policy (CAP) 2021-2027 [4] with main targets of reducing the overall annual GHG emissions and encouraging the usage of energy produced from renewable sources. CHRSs comply with these targets directly, as these systems can result in the reduction in GHG emissions and advance the benefit of renewable energy, reducing the reliance on heating systems based on fossil fuels as they can capture and reuse thermal energy produced from computing systems [5]. In addition, the CAP 2021-2027 further underscores the importance of sustainable agriculture practices and the usage of renewable energy in rural areas. The adoption of CHRSs will encourage sustainable organic waste management and GHG reduction, as well as providing renewable energy sources up to the farm level, which will support agricultural practices to meet the larger sustainability and climate change mitigation trends expected from the EU. A recent study in Northern Italy described the performance of a large-scale CHRS fed with tree-pruning residues [5]. The research findings indicated that the heat supply costs of these systems were competitive when compared to fossil fuel sources while also leading to carbon emission reductions. The cost per unit of energy from the system was 0.087 EUR/kWh, which decreased to 0.074 EUR/kWh with the installation of two CHRS units. Moreover, it was observed that significant carbon savings up to 0.252 kgCO\({}_{\text{2-eq}}\)/kWh could be achieved by saving energy and promoting carbon sequestration in lands. This emphasizes the advantages of employing CHRSs for energy recovery and as a climate change mitigation strategy. CHRS applications contribute to the achievement of part of the United Nations Sustainable Development Goals (SDGs) [6]. Specifically, CHRS provides renewable energy that is clean and environmentally friendly, contributing to SDG 7 (Affordable and Clean Energy) by providing a sustainable energy source while reducing reliance on fossil fuels. Additionally, the reduction in greenhouse gas emissions due to compost heat recovery contributes to SDG 13 (Climate Action) by mitigating climate change impacts.