Use of Air Cooled Condenser in Biomass Power Plants: A Case Study in Cuba

At the present time, the deficit tried of water and the urgency of the use of the alternative sources of energy, have generated important efforts channeled to solve the existing deficiencies in the used technologies. The use of biomass as an energy source for generating electric power has been one of the most widely accepted alternatives in regions with agricultural and forestry potential [1].

As part of the strategy drawn up by the Cuban state in terms of energy and hydrological sustainability, in the five-year period 2020-2025, an appreciable group of investments are executed in the country, with the purpose of increasing the presence of renewable sources in the national matrix of energy. These include a total of 25 Biomass Power Plants (BPP) that will be associated with the same amount of Sugar Power Plants (SPP) currently in operation, the latter becoming a source of fuel biomass supply (bagasse and cane agricultural waste), being 20 and 50 MW base powers used [2].

However, the current location of the SPP is an aggravating element for the start-up of the BPP, since there are no nearby water sources that are capable of covering the flow rates required by the condensation systems, (approximately 160 m³/h). In the dry season (2019), 37 water basins in Cuba were declared as critical state, reducing the capacity of delivery to the minimal. This situation evidenced that Cuba is not exempt from the global water crisis [3].

According to the report [4], at the end of 2017, 32% of water withdrawals for industrial purposes were destined for wet condensation systems. In order to reduce the consumption of water in power plants, the use of the so-called dry condensation is gaining ground, because as its name indicates it dispenses with the consumption of water for its operation, achieving savings rates close to 98% with regarding wet condensers [4, 5].

Dry cooling systems have the potential to almost eliminate the use of water in the BPP. Among the dry condensers, one of the most widespread is the so-called air-cooled condenser  (ACC), being already known and used in the BPP located in countries such as the United States, Turkey, China, Malaysia, India, South Africa, Germany and Spain, although it has not yet been widely disseminated, since it barely covers 1% of current BPP, as proposed by Huang et al. [6, 7].

However, the ACC have achieved limited penetration in power plants, due to considerable compensation in terms of cost and performance, as they require a capital investment substantially greater than wet condensers because they incorporate larger heat exchangers, with huge fin areas and require additional support structures [8].

In general, the installation and operation costs of the ACC systems are currently 2.5 to 5 times higher than their wet equivalent, while the typical costs of level energy production for plants with ACC range from 40 to 80 USD/MWh, being approximately 15% higher than the costs obtained with the use of wet cooling technology [9, 10].

In the existing and available literature, similar experiences are not reported in areas with operational and climatological similarities to national ones, so issuing a judgment on the feasibility of the possible use of ACC would require a case study in which they were simultaneously considered several condensation technologies, in order to establish initial investment cost and life cycle levels. To demonstrate the viability of the use of ACC in the projects of BPP planned in the country is the objective of the present paper.

3.1 Comparative criteria of the selection matrix for the initial investment

The movement of funds of an investment consists in determining in each one of the periods in which the horizon was divided, how many collections and how many payments are made. The analysis is done by balancing inputs and outputs. Without a fund movement, it is not possible to evaluate an investment, so it is necessary to carry out a preliminary market study, which allows including all the possibilities of offers. However, in this work, only one supplier is used, since due to the restrictions imposed on the Cuban state by the economic-commercial blockade, four suppliers consulted only receive a response from HOLTEC INTERNATIONAL. This work complies with the provisions of the current investment resolution in the country (Decree No. 327-2015).

Table 4. Operating costs according to the Kaplan method

All initial equipment costs, (factory inspection, technical assistance, import duty, freight, insurance, basic engineering and inspection at final destination port), were obtained in direct communication with ENERGOIMPORT, the only authorized entity in Cuba for to import facilities destined for the energy industry. The current external financing available to this entity is of Chinese origin, with a bank interest of 5.5% and an update rate of 10%. In the fruitful consultation made [16], the useful life period for the four variants of technologies analyzed is established, being equal to the 25 years for wet technologies and 35 years for dry technologies, taking a 20-year horizon to affect the movement of funds.

3.2 Initial system balance

The average unit costs in USD/kW for various condensation technologies were obtained in the consultation made to HOLTEC, these being considered current when acquired directly from the supplier with update date 03/2019. A summary of these costs is provided in Table 5.

To update equipment costs for periods other than the preparation of this report, you can go to the Marshall & Swift Equipment Cost Index (M&S), the most accepted cost index rate among the main suppliers of condensation systems according to the paper [19]. This rate is described by:

${{V}_{MS}}={{V}_{AA}}\cdot \left( {{{I}_{11}}}/{{{I}_{AA}}}\; \right)$                (11)

where, V_AA is the value of available equipment cost, in MUSD; I₁₁ is the Marshall index on the date it is intended to assess the cost; I_AA is the Marshall index of the date that the equipment cost is available.

Table 5. Cost for different condensation technologies

The heat flow to be evacuated for each operational situation is detailed in Table 8, while, in the Table 9 are given the water flow required for wet condensers in each variant analyzed. For both powers, the variant considered as critical is variant 1, as it includes the states of maximum operating requirements, and therefore, the case study will be based on its basis.

Table 6. Indexes Marshall & Swift Equipment Cost Index

Several selection criteria are used in the evaluation of the four variants of technologies analyzed in this study, these criteria are:

1. Pay off period
2. Interest rate of return (IRR)
3. Net present value (NPV)
4. Life cycle cost
5. Level energy cost
6. Cost benefit ratio.

Due to the volume of information and variables involved, the results obtained for the BPP of 20 MW and 50 MW are summarized and presented in Tables 10 and 11. Here, load factors and losses equal to 0.72 and 0.58 are taken respectively, an average cost of energy sales of 0.127 USD/ kWh, as well as 19.1 equivalent hours of charging, as stipulated by the research [11]. The mitigation costs are equal to the sum of costs of emissions, operational pollution and cooling water consumption. Previously it was given the criteria of several authors in which they establish the levelized cost of energy for ACC between 40 to 80 USD/MW. The results obtained in this work are located in this range [20-23].

Table 10. Summary of the case study for a 20 MW BPP

Table 11. Summary of the case study for a 50 MW BPP