Solar insolation data have been recorded in Hawaii from as early as 1932, at 140 observation stations over various periods. Almost all of the data were collected by the sugar industry, which maintains a network of roughly 100 stations located on the four sugarcane-growing islands, for the evaluation of solar energy available for photosynthesis. The use of several types of instruments limits the accuracy of radiation observation to 10% (How, 1978). Because the data are used for agricultural purposes, only global insolation is collected and reported.
Scientists at the University of Hawaii have also monitored solar insolation values for a number of years. With the onset of the oil crisis, renewable energy became an important issue for Hawaii because of its natural energy resources and heavy dependence on petroleum. Background data on solar insolation were collected at various sites. Additional sites were added through USDOE funding, but the nature of the program and availability of equipment restricted the information largely to global data monitoring (P. Ekem, personal communication, 1986; Falicoff, Koide, and Takahashi, 1979; Law, 1976; Yoshihara and Ekem, 1977,1978).
Fig. 1 shows the annual average global insolation values for various islands. Direct insolation data are limited to only a few stations and were primarily calculated from the global and diffuse components. The figures also show the direct insolation measuring stations with average annual values in parentheses.
High wind regimes also indicate potential cloud cover and rainy areas. Prevailing trade winds from the northeast usually push clouds toward the islands, and the clouds then accumulate at the mountain ranges and deposit their moisture. Thus, the leeward sides of the islands are dry, cloudless, and sunny. While south winds sometimes deposit rain in these dry areas, these conditions occur only for brief periods during the year.
Sufficient direct normal insolation, adequate land area, and end uses of the solar-generated energy are important factors to consider in developing solar thermal systems. Based on existing global and direct insolation information, land availability, presence of end-users, and transmission, areas for potential application have been identified (shaded areas in Fig. 1). Direct insolation is the key factor because it is not available or measured throughout the state. Land availability is important, especially in Hawaii, because space is so limited. Lastly, a market or need must be available or established for the system product. Electricity is, by far, the easiest product to market with available transmission lines; however, other applications such as process heat (e. g., sugar mill operations), heating, desalination, or others (space cooling and dehumidification) could prove feasible.
The Natural Energy Laboratory of Hawaii (NELH) was created by the Hawaii Legislature in 1974 as a facility for natural energy research and is an ideal site for concentrating solar thermal systems. While 1982 presented an uncharacteristically low direct insolation (about 98 W/m2), which can be explained by it being the wettest in the last 50 years and the year that Hawaii was hit by Hurricane Iwa (Yuen, Seki, and Curtis, 1983), further data collection has shown it to be a very good site (160-175 W/m2). NELH is also the site of various renewable energy projects such as open-cycle ocean thermal energy conversion and peripheral aquaculture activities, not only on the 1.31 km2 site occupied by NELH, but the neighboring Hawaii Ocean Science and Technology (HOST) Park’s 2.21 km2. With the recent development of the Kona district, electricity and water are needed. Land is generally available, but on barren lava rock, which could increase site preparation cost. The NELH and HOST park facilities are adjacent to the Keahole Airport, thus there are height restrictions near and around the surrounding area, which obviously could prevent power tower systems.
On Kauai, Lihue on the southeast part of the island, has the only direct insolation measuring station. Solar potential, however, exists in the southern and southwestern sections of the island and the Wilcox Hospital concentrating solar system was relocated to the west side. Sugarcane growing and grazing dominate the use of this agriculture-zoned area. Water is a primary concern in the area.
On Maui, the best areas for concentrating solar thermal systems are in the valley or plains and upper elevation of Haleakala, where global insolation is excellent. Since there are several power generating facilities in the valley area, transmission lines for interconnection are available. The proposed Maui Renewable Resources Research Facility and Maui High Tech Park Facility, sponsored by the state and county governments, are under development. The laboratory will be a test bed for renewable energy research and development and a possible site for concentrating solar thermal systems. A key consideration in designing any such unit at this site is the wind load. The two Maui mountain groups, Haleakala and the West Maui Mountains, tend to funnel winds through the valley, making it an excellent site for wind energy; however, strong winds can be detrimental to concentrating solar thermal system structures. Appropriate design in the solar systems could help prevent any problems. The Kahului Airport is located in the northern section of the valley or plains, thus imposing height restrictions on wind developments. The Lahaina area of West Maui has excellent direct insolation for concentrating systems, according to recorded data (Maui’s only station at the Pioneer Mill has an annual average of 181 W/m2). Lahaina was the site for the proposed solar repowering project, mentioned earlier. Inasmuch as this area is experiencing resort development, high land costs could pose a problem.
On Molokai, a potential site for concentrating solar thermal systems includes the site for the now defunct Small Community Solar Experiment. This south-central section of Molokai has high wind insolation (171 to 206 W/m2). Molokai is the least-developed of the five largest islands. Residential electricity rates average about 20 cents/kWh and the unemployment rate is the highest in the state over the past 10 years, about 14%. Thus, any development that has the potential for job creation and the reduction or stabilization of electricity rates could be appealing to the community. Water for irrigation is a problem; currently, it is being transported through tunnels from the wet eastern mountain region to a 5.67 million m3 capacity reservoir and distributed to the low-lying farm lots in the central region. The Kaunakakai Airport is located in central Molokai. Thus, depending on the actual location of the concentrating solar thermal system, there may be height limitations on central receiver units. Since Molokai Electric has only a 5 MW capacity, any large solar thermal system developed greater than the present capacity will probably have to be transported to the population centers (Maui and Oahu) via an undersea transmission cable.
On Oahu, the western area from HECO’s Kahe power plant up the coast to Makaha is generally quite dry, sheltered from the prevailing trade winds by the Koolau and Waianae mountain ranges. Limited direct insolation measurements at Kahe Point have shown high insolation with an annual average of 163 W/m2. Open space is favored in this rural community, thus solar thermal development may be difficult.