Cost of centralized versus de-centralized systems

Cost of Collectors Vs System Cost. When comparing solar thermal system alternatives: system size, configuration, types of collectors, etc., all cost components over the full lifecycle of the system should to be taken into account. This requires detailed information on the system hardware and results of system’s performance simulation.

Advanced technology and production economies of scale have led to significant cost reductions in solar hot water collectors. The value of shipped low-temperature collectors was $1.89/sq ft (~14 €/m2) in 2008. The average cost of thermosyphon systems with the storage integral to the collector was $24.27/sq ft (~183 €/m2) ; the price of flat-plate collectors was $17.40/sq ft (~131 €/m2); the price of evacuated tube solar collectors was $25.69/sq ft (~194 €/m2); and the price of parabolic trough solar collectors was $11.96/sq ft (90 €/m2) These values are based on collector factory revenue divided by output, so retail prices would roughly double (not including labor for installation), and the installed system price with all the other components is on the order of $75 to $225/sq ft (565 to 1696 €/m2) depending on project size and location. New construction systems usually have better economics than retrofit projects because of reduced installation expenses.

Independent evaluation of a single system component cost and its performance, e.g., solar collector, provides limited information for the whole system evaluation. Increasing the size of a single component (e.g., the collector area or the storage tank volume) results in “diminishing returns.” This means that, although the yield will increase when the collector area or storage tank volume is increased, the marginal increase becomes smaller with increasing area and volume. Also, maximizing one component over others has a strongly diminishing effect, which usually results in increased energy costs ($/kWh). Simulation of the system with variation of component sizes aid the design and energy cost optimization. Figure 4.6 shows approximate composition of the large system first cost.

Large central solar water heating systems are normally more cost effective due to economies of scale in installation, operation and maintenance compared to several small systems. This includes higher efficiencies possible* for backup heating systems in centralized systems. Table 4.4 lists the approximate effect of the solar thermal system size on investment cost generated from analysis of case studies described in Section 4.5 (p 71).

System efficiency factor

Losses in heat distribution increase with distance and have an additional negative effect on overall system efficiency. For example, for large centralized systems the required storage temperature and delivery temperatures increase (above the operating temperature) due to heat losses through the walls of a storage tank and in the distribution system. Simulations of specific system is required to determine the overall performance (e.g., distribution losses as a fraction of the energy supplied).

Operation

In the particular situations with high peak loads and long periods when there is no hot water use in some buildings (e.g., in barracks during soldiers’ deployment) central system will simplify adjustments needed to control such situations and potentially reduce the damage to the system. In the case of compounds with multiple barracks, large central SWH system can be more easily installed, maintained, and operated. Even finding a location for the solar collectors may be simpler. Large SWH systems also have the advantage in that the influence of individual users is minimal on
their operation.

Small SWH systems require periodic checking and maintenance. Scheduling of maintenance for many distributed systems may be an issue of prioritization. On the other hand, maintenance of large central systems is more critical since a larger number of users could be affected and thus system monitoring and preventative maintenance are required activities to assure good performance.

Storage

Thermal losses from the storage tank have a significant effect on the efficiency of the solar thermal system. Heat is lost via the surface. The capacity of a sensible storage tank (e.g., water steel tanks) is defined by its volume. The volume grows with the 3rd potency of its circumference. The surface grows with the 2nd potency. Thus storage tank losses are reduced when one large store is used in comparison with many small storage tanks.

Back-up /auxiliary heating

Each solar thermal system must have a back-up heating supply. Oil, gas, electricity, or biomass is used as a heat source. Due to the 20+ year lifecycle of heating systems, including solar thermal systems, possible changes in energy supply options should also be  considered. This could, for example, relate to future auxiliary (back-up) heating options from biomass (bio-waste) and waste heat supplies. In the case centralized systems, these options are often more easily integrated later.