Engineering task and calculation objective
The KPRE module compares the economics of dry cooling and wet cooling for power plant and industrial cooling systems. From the capital and operating costs it determines the limiting price of cooling water, i.e. the price of make-up water at which both cooling methods are equally expensive: if the actual water price at the site is above this value, dry cooling pays off despite its higher investment; if it is below, wet cooling remains the more economical choice.
The calculation includes the governing cost variables of both systems: the heat price of the fuel, because dry cooling causes higher condensation temperatures and thus efficiency losses; the specific additional plant cost for dry cooling; the annuity plus the costs for repair and maintenance; and the annual utilization period at nominal load. On the wet cooling side, the specific mean annual evaporation loss and the blowdown are taken into account, which together determine the make-up water demand.
Such a comparison is needed for site and concept decisions for recooling plants, especially where water is scarce, expensive or restricted by permits. The module delivers a robust key figure for preliminary planning, before detailed quotations for a cooling tower or an air-cooled condenser are obtained.
Calculation workflow
- Define the operating profile: First the energy-economic boundary conditions are recorded: the annual utilization period at nominal load as a measure of capacity utilization, and the heat price of the fuel, with which the additional consumption caused by the poorer cooling level of dry cooling is valued.
- Assess the additional costs of dry cooling: The specific additional plant cost of dry cooling compared with wet cooling is converted into annual costs using the annuity plus the costs for repair and maintenance; added to this are the fuel-side additional costs resulting from the higher condensation temperature.
- Determine the water demand of wet cooling: For wet cooling, the make-up water demand is determined from the specific mean annual evaporation loss and the blowdown that is required to limit the concentration of the circulating water.
- Calculate the limiting price of cooling water: From the balance of the annual costs of the two options follows the limiting price of cooling water: the annual additional costs of dry cooling, referred to the make-up water quantity saved compared with wet cooling.
- Compare with the site price: The calculated limiting price is compared with the actual supply and disposal price for water at the site; from this follows the recommendation for dry or wet cooling and the sensitivity to fuel price and capacity utilization.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Dry cooling versus wet cooling | dW | J/kWh |
| Fuel costs | wb | EUR/J |
| Additional plant cost for dry cooling | dK | EUR/kW |
| Annual costs plus service etc. | a' | 1/a |
| Annual nominal load | bN | h/a |
| Mean annual evaporation loss | mWO | kg/kWh |
| Blow ratio for wet cooling | μF | – |
| Limiting price of cooling water | KWG | EUR/kg |
Frequently asked questions
Why does dry cooling cause additional fuel costs?
A dry cooling system cools against the dry-bulb air temperature, whereas a wet cooling tower, thanks to evaporation, cools against the lower wet-bulb temperature. Dry cooling therefore reaches higher cooling water and condensation temperatures, the condenser pressure rises and the efficiency of the steam power cycle drops. This loss is converted into annual costs via the heat price of the fuel and the utilization period, and set against the water costs of wet cooling.
What is blowdown and why does it belong in the water demand?
During evaporation, the entire salt content remains in the circulating water and concentrates it. To limit deposits and corrosion, a partial stream is continuously blown down and replaced by fresh water. The make-up water demand is therefore the sum of evaporation loss and blowdown; the poorer the water quality, the higher the required blowdown rate and the more expensive wet cooling becomes.
How should the limiting price of cooling water be interpreted?
It is the break-even price of the make-up water: if water at the site, including treatment and wastewater disposal, costs more than the limiting price, dry cooling is cheaper overall; otherwise wet cooling is. Since fuel price, capacity utilization and interest rate have a strong influence, the limiting price should always be calculated with ranges of these parameters, not just as a single value.
Does the comparison also cover hybrid cooling?
The classical limiting-price comparison sets pure dry cooling against pure wet cooling. Hybrid systems, which spray water in summer and otherwise run dry, lie between the two options; they can be assessed approximately by inserting their partial water consumption and additional costs into the same annual cost calculation. For a concept decision including a hybrid option, a separate analysis is advisable.