Techno-economic analysis of multipurpose OTEC power plants

. Ocean Thermal Energy Conversion (OTEC) is a promising technology to provide sustainable and dispatchable energy supply to oceanic coastal areas and islands. It exploits the temperature difference between deep cold ocean water and warm tropical surface water in an Organic Rankine Cycle (ORC), guaranteeing a continuous and dispatchable electric production, overcoming one of the most critical issue of renewable generators such as PV or wind turbines. Despite the technological maturity of ORC application to OTEC systems, it still presents technical and economic barriers mainly related to their economic feasibility, large initial investments as well as heavy and time demanding civil installation works. To overcome such issues, multipurpose OTEC plants are proposed, producing electrical power as well as other products, such as useful thermal power (e.g. ambient cooling) and desalinated water. Since OTEC engineering is still at a low degree of maturity, there are no widespread and established tools to facilitate OTEC feasibility studies and to allow performance and cost optimization. Therefore, in this paper, a new tool for techno-economic analysis and optimization of multipurpose OTEC plants is presented. Starting from a detailed database of local water temperature and depth, the approach allows to provide a quantitative insight on the achievable performance, required investment, and expected economic returns, allowing for a preliminary but robust assessment of site potential as well as plant size. After the description of the techno-economic approach and related performance and cost functions, the tool is applied to an OTEC power plant case study in the range of 1 MW gross electrical power, including a preliminary assessment of scaling-up effects.


Introduction
In recent years, energy independency of islands and archipelago has become an important issue for many researchers, particularly analyzing their renewable potential and the possibility to test innovative concepts such as smart grids, energy storages, new power plants and energy efficiency solutions [1]. Among those, Ocean Thermal Energy Conversion (OTEC) technology boasts competitive advantages over alternative sources of electricity production regarding accessibility, predictability, dispatchability and emission.
OTEC consists of a process that can produce electricity by using the temperature difference between deep cold ocean water and warm tropical surface water. OTEC plants pump large quantities of seawater to run a power cycle and produce electricity 24/7. Energy produced by OTEC plants can be considered dispatchable providing a reliable energy sources for not connected to National Grid islands [2].OTEC also offers the possibility of cogenerating other synergistic products, like fresh water, nutrients for enhanced fish farming and seawater cooled greenhouses. Cold water can be used also in building air-conditioning systems. Energy savings of up to 90% can be realized [3]. Careful site selection is a crucial point to keep the environmental impact of OTEC minimal and ensure economic sustainability. Researchers believe that appropriate spacing of plants throughout tropical oceans can nearly eliminate any potential negative effects on ocean temperatures, local ecosystems and marine life [4]. Another factor hindering the commercialization of OTEC is that there are only a few hundred land-based suitable sites in the tropics, where deep-ocean water is close enough to the shore [5]. Therefore, techno-economic tools for site assessment in view of OTEC installations are strategic to reduce survey time, design costs and to allow identifying the locations where energy source and need are well combined. In this paper, a new tool is presented to analyze the techno-economic feasibility of multipurpose OTEC power plants, starting from real monitored seawater data, in order to evaluate the potential of this technology in a straightforward but complete way. The tool is the result of a fruitful collaboration between WAVEC, with its experience in monitoring seawater and map its energy potential [6,7], and the University of Genova, Thermochemical Power Group (TPG), with its background in thermoeconomics and energy district analysis [8,9].

Techno-economic simulation tool
Since OTEC technology is still under development, to overcome the lack of field data, some technology providers such as Bluerise [10] and Makai [11] have developed tools for a preliminary assessment for the Ocean Thermal Energy. However, there is not available yet a comprehensive tool, not based on a specific technology, for the preliminary techno-economic assessment of OTEC installations.
The tool, developed in the MATLAB®-Simulink® environment, provides steady-state thermo-economic analysis of a land-based Closed-Cycle OTEC plant, working with ammonia, along with optional devices (SeaWater Air Conditioning, desalination plant, etc.). Since these optional products, i.e. air conditioning and fresh water, are considered fundamental for the economic sustainability of OTEC technology in the near future, only land-based plants have been considered, disregarding floating plants, where multi-purpose OTEC systems will be less likely realized.

Figure 1:
Closed-Cycle OTEC system layout [12] Closed-cycle OTEC system [13] can be considered as the "traditional OTEC" plants. This cycle uses a low boiling point fluid, such as ammonia or other organic fluids, to operate a traditional Organic Rankine Cycle (ORC) plant to generate electricity [14]. Warm surface seawater is pumped through a heat exchanger, where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Cold deep seawater, which is pumped through a second heat exchanger, condenses the vapor back into a liquid that is then recycled through the system. The advantages of using a closed-cycle system are compactness and that it can be designed using already existing turbo machinery and heat exchanger designs. The model incorporates worldwide bathymetry from the NOAA database [15] and seawater temperature monthly maps from the Copernicus database [16], depending on latitude, longitude and depth. This allows to perform techno-economic assessment without any site restriction. The tool receives multiple parameters on the inlet and outlet pipelines, and it carries out a preliminary design of the whole plant, calculating seawater and ammonia mass flows, thermodynamic data of the ORC and its efficiency, etc. Furthermore, it also performs a preliminary plant economic. The simulation process flow diagram of the tool is summarized in Figure 2. To capitalize on the discharge cold water of the OTEC plant for other purposes and consider multipurpose OTEC concepts, a SeaWater Air Conditiong (SWAC) system is included, which represents a proven technology that uses the cold seawater to cool commercial buildings or residential complexes close to the sea (or lake). The operating variable costs of the plant are nearly independent from future energy price increases, since SWAC requires electrical power that can be directly provided from the OTEC system.
To compute the thermal load required by the SWAC system, three main sources have been considered: 1. Thermal load due to people in the building, lights, electrical devices 2. Specific heat due to the temperature difference between indoor and outdoor air temperature 3. Specific heat through glassy surfaces due to thermal solar irradiation. To perform then a preliminary economic analysis of the plant, several correlations were developed from industrial reference capital costs [17][18][19][20][21][22]. The user has the possibility to insert an hourly profile of the electrical power load of the facilities, so that the hourly electrical power that can be sold to the grid can be determined as the difference between the power produced by the ORC and the one consumed by the pumps, considering as well an electrical generator efficiency of 98%.

Case Study
The techno-economic simulation tool has been used for assessing the energy and economic potential of the following site and multipurpose OTEC plant: • Location: (longitude: -81.106, latitude: 19.297), close to East End, Cayman Islands • ORC turbine size: 1 MW Given the worldwide database available in the tool, the plant location can be chosen in any shoreline. For this case study, the plant has been located in order to have good working conditions for OTEC plant (the sea bed around Cayman Islands presents a high slope and so it is possible to have a large difference of temperature between surface and deep water ( Figure  3) without going too far from the shore) which would highly affect the cost of the pipeline. The size of the ORC turbine, 1MW, has been assumed to be in line with the current on-shore working plants and the constraints of the local electrical grid. The approach temperatures at the evaporator and condenser have been fixed in order to obtain the best working solution of the plant: this was verified through iterative simulations, not reported here.
The SWAC system has been coupled to a building with the following characteristics:  From the results, it has been observed that the best case appeared to be for a ΔTcond of 4.5°C; therefore, this value was fixed for the results presented in the following. The overall capital cost (not including discharge pipeline, civil works, installation, land, etc.) can be divided as shown in Figure 6. As expected, pipeline to convey water to the process block is one of the most expensive components of the OTEC plant, even in case of proximity with deep cold water. High costs of seawater pumping systems are related to the extremely high mass flow rates with which the plant operates. On the other hand, SWAC has a limited impact on the overall costs (promising option to increase investment profitability).

Figure 6: Distribution of capital costs
As it can be seen, even in February the total efficiency of the cycle is good compared to the Carnot one, thanks to the difference of temperature between cold and warm seawater that is higher than 20 degrees, making the selected location a suitable place for the installation of a multi-purpose OTEC system. The plant behaviour is now evaluated during the whole year, in order to evaluate the effects of the variation of the boundary conditions. The most relevant results are reported in the following table: Highlighting the variation of the net power produced by the plant along the months, it can be observed that the highest net power occurs in the months between June and October.

Large size OTEC plants
In order to evaluate the impact of size on OTEC plant economics, two additional different sizes are considered, for the same installation site: 5 and 10 MW gross power. The best design values, previously identified, have been considered (i.e. ΔTcond = 4.5°, Flow velocity within pipes = 2 m/s). A comparison between the three plant sizes is summarized in the following table: As expected, increasing the size of the plant is beneficial from the specific capital cost point of view (M$/MW). However, the capital costs for large power plants remain high at the current state of the art, and the pipe large diameter may lead to possible issues for near-coast installation.

Conclusions
In this paper a new simulation tool for the techno-economic analysis of multipurpose OTEC plants has been presented. The main goal is to provide a preliminary evaluation of the thermodynamic and economic performance of the OTEC plant, starting from monitored data of seawater conditions in terms of temperature and depth in a specific installation site.
The tool provides the optimal plant design point as well as the yearly performance, starting from monthly data of seawater. Such an analysis is representative of a higher resolution assessment due to two main facts: 1) The stability of water temperature during a month that guarantees the OTEC plant not to work in strong off-design conditions day-by-day 2) The almost stable power production of the OTEC plant that can guarantee the baseload as it can work 24/7. The modeling approach has been introduced, highlighting its capability to evaluate multipurpose OTEC plants: in fact, polygeneration of power, heat/cooling and desalinated water can be considered crucial for the development and spread of OTEC plants, which are still affected by high capital costs, mainly related to the seawater piping and pump system. In particulare, desalination and Sea Water Air Conditioning are two potential by-products that can guarantee additional revenues to OTEC plants The tool will enable OTEC designers and investors to have a preliminary fast but reliable evaluation of the technical and economic feasibility of an OTEC installation, starting from local marine data. A case study has been analysed in detail for a 1MW gross electrical power OTEC, with the following results: -The efficiency of the cycle along months is around 55-60% of the Carnot one, achiving the maximum in August at ηmax= 3.22%