Potential of ORC Systems to Retrofit CHP Plants in Wastewater Treatment Stations

Abstract

Wastewater treatment stations take advantage of the biogas produced from sludge in anaerobic digesters to generate electricity (reciprocating gas engines) and heat (cooling water and engine exhaust gases). A fraction of this electricity is used to operate the plant while the remaining is sold to the grid. Heat is almost entirely used to support the endothermic anaerobic digestion and a minimum fraction of it is rejected to the environment at a set of fan coolers. This generic description is applicable to on-design conditions. Nevertheless, the operating conditions of the plant present a large seasonal variation so it is commonly found that the fraction of heat rejected to the atmosphere increases significantly at certain times of the year. Moreover, the heat available in the exhaust gases of the reciprocating engine is at a very high temperature (around 650 ºC), which is far from the temperature at which heat is needed for the digestion of sludge (around 40 ºC in the digesters). This temperature difference offers an opportunity to introduce an intermediate system between the engines and the digesters that makes use of a fraction of the available heat to convert it into electricity. An Organic Rankine Cycle (ORC) with an appropriate working fluid is an adequate candidate for these hot/cold temperature sources. In this paper, the techno-economic effect of adding an Organic Rankine Cycle as the intermediate system of an existing wastewater treatment station is analysed. On this purpose, different working fluids and system layouts have been studied for a reference wastewater treatment station giving rise to optimal systems configurations. The proposed systems yield very promising results with regard to global efficiency and electricity production (thermodynamically and economically).

A Humid Air Turbine–Organic Rankine Cycle combined cycle for distributed microgeneration

“…Although this is a non-commercial technology, the equipment costs of the HAT cycle components have been calculated using the approximations presented in [46,47], with reference values from microturbine and ORC technologies. Each one of the ORC cycles is estimated globally with regard to their nominal power output as described in [48]. The financial and economic indicators used for the analysis are presented in Table 6.…”

Energy analysis of products and processes in a sanitary landfill

“…Biogas can be utilised in boilers [6], in direct thermal applications [7,8] such as ovens and sludge dryers, for the evaporation of leachate [9,10], for electricity generation using gas engines [11,12], hybrid organic Rankine cycle systems [13] or fuel cells [14,15], as cooking gas [16], or as fuel for vehicles [17][18][19]. Advances in these applications are affected by the assessment of LFG production capacity [20,21], regional characteristics of the landfill [22][23][24], effects of new technologies for the improvement of biogas generation [25,26] and changes in laws and taxes [19,27].…”

Modeling of a Cogeneration System with a Micro Gas Turbine Operating at Partial Load Conditions

“…On the other hand, Chacartegui [8] has published a study about the potential of organic rankine cycle systems to be used as retrofit cogeneration plants in wastewater treatment stations.…”