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									POWER AND ENERGY

THERE’S SOMETHING IN THE AIR
By Zoltán Szabó

ENVIRONMENTALLY FRIENDLY COOLING SYSTEMS FOR ALL SEASONS

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Figure 1

ith increasingly stringent environmental laws coming into place, many power plant operators are looking for ways to comply with these new regulations. But the associated cost of compliance can be high. We look at the different options available and the pros and cons of each system from both an environmental and economic point of view.

Heller & evaporate

Head Plume abatement hybrid All wet 0% 50% Annual water consumption relative to that of an all wet cooling Water conservation features of different cooling systems 100%

Heller sprayed All dry Heller

The 50m high twin flue gas stacks within the Heller type dry cooling tower shell substituting a high chimney at Can TPP

WET (EVAPORATIVE) COOLING TOWER
At the maximum end of the water consumption scale is the wet cooling tower – the most frequently used power-cooling system. It has the lowest capital cost and provides the best vacuum, however, it needs a significant amount of make-up water (a 100MW power plant uses as much water as a town of 50,000 inhabitants). In addition, wet cooling towers emit vapour plumes with the related drawbacks and discharge of concentrated cooling water blowdown, which contributes to environmental pollution. The choice of cooling system has an effect throughout the life of the power plant. The existence of proven water conserving cooling systems – in parallel with ever more stringent environmental regulations – underlines the importance of making a comprehensive economic analysis in selecting the optimal cooling solution. There is a remarkable tendency – even in areas where power plants had approval to use water for evaporative cooling – for water shortage, in time, to become a significant constraint to the full capacity of these plants. Water shortage may reach such a level that the high make-up water price justifies a transition to a water conservationtype cooling system.

rejection. The all dry cooling plant, such as EGI’s Heller System – emitting only warm and clean air – has no adverse environmental effects and, at the same time, frees power plants from dependence on water sources. The Heller system is an indirect dry cooling plant in which the power plant’s waste heat is initially exchanged, preferably in a direct contact condenser (instead of a surface one), to a closed cooling water circuit. The heat absorbed by the water is rejected to ambient air in fin-tube type heat exchangers. Either mechanical or natural draft can be applied. The capacity of power plants equipped with the Heller system has already exceeded 17,000MW. Although the capital cost of dry cooling systems is higher than that of evaporative cooling, it is worth remembering that in addition to eliminating the need for cooling water, the advanced Heller system (especially its natural draft version) significantly reduces the cooling system maintenance costs, improving the power plant availability. Current detailed present value based life-cycle evaluations show how the HELLER system could extend the economic viability of dry cooling against wet cooling.

DRY/WET COOLING SYSTEMS DRY COOLING: HELLER SYSTEM
EGI has developed pioneering solutions for water conservation and environmentally friendly alternatives for power plant heat Dry/wet cooling system combinations should be considered over all wet cooling if there is an emphasis on reducing the environmental impact, or if the available water is less than needed

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for an all evaporative cooling system and/or the make-up water price exceeds a certain value, but still doesn’t justify dry cooling. Application of dry/wet cooling system combinations offers a further opportunity to increase the share of water conservation type cooling systems. A number of the existing combinations aim at improving aspects of the completely wet or completely dry cooling towers: • Improving environmental compatibility and water conservation features relative to wet cooling. • Improving summertime turbine back pressure (power generation) and reducing investment costs, relative to dry cooling. EGI has developed several dry/wet solutions based on its Heller system aimed at saving a substantial quantity of cooling water, while avoiding a modification of the steam turbine. These technical variants offer adequate dry/wet solutions for areas of varying water resources for either completely new power plants or existing plants to be converted from all wet systems.

Mechanical Draft Heller System with optional supplementary spraying, Japan

DRY SYSTEM WITH SUPPLEMENTARY WATER SPRAYING
A dry Heller system can be equipped with optional water spraying facilities to be used for ‘peak-shaving’ in the summer days. Time wise, the extent of spraying is limited by the importance of avoiding extensive scaling on the heat exchanger outside surface (suitable fin tubes try to do the same job). This solution, first of all, serves the optimal matching of ‘cold-end’ (LP turbine and cooling system) and makes derating avoidable even in the hottest summer days. Year-round, the water requirement is 1-10 percent that of an evaporative cooling tower. Therefore, from the point of view of water conservation, it approaches the all-dry cooling systems, however, it can support higher power output in summer.

HEAD (DRY/DELUGED) COOLING SYSTEM
A further well-proven solution is the HEAD Cooling System. The system operates as dry for a significant part of the year except during the summer hours coinciding with peak power demand. Then, an even water film (deluging) is applied on the special plate fins of the air-cooled heat exchangers. The applied quantity of

Plume abatement hybrid cooling tower with noise attenuation

PLUME ABATEMENT HYBRID COOLING
The so-called Plume Abatement Hybrid Cooling System is essentially a wet cooling tower with a dry section incorporated into the same cooling water circuit. When the dry section is in operation, the water of the wet cooling tower is circulated in the fin tubes, which require the use of relatively expensive stainless steel material to withstand corrosion. Thus, the size of the dry section is limited by economic considerations. Also, the dry section is susceptible to clogging and deposition leading to increased maintenance costs. The primary target is to avoid or at least reduce visible plume during winter therefore, the cooling air of the wet and the dry sections should be properly mixed prior to exhausting from the tower. During the summer period, the cooling system is normally operated as an all wet tower bypassing the dry section. Limiting the operation time of the dry section reduces the necessary maintenance. This is not really a water conservation type system, since it consumes, year-round, about 94 percent of the water required for a wet cooling tower.

HEAD-type dry/wet cooling system for a 150 MW combined cycle co-generation plant in Budapest

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water is much more than the amount of evaporation, therefore, the excess water is collected and re-circulated after blow-down and addition of the necessary make-up. It results in a major improvement in summer plant performance at a lower investment cost than that of an all-dry system. Meanwhile the make-up water used on a year-round basis is not higher than 30 percent of an all wet tower. This is a solution ideally suited to summer peaking power generators or co-generation plants with seasonally changing heat load. An interesting variant occurs when a large all-dry natural draft-cooling tower is supplemented with mechanical draft dry/deluged HEAD cells to enhance summer capability. These cells can be located either outside or inside the tower. In the latter case, for plants operating in areas of severe winter climate, the same cells can be used as so-called pre-heaters during the start-up period, ensuring a freeze-proof start even under the most unfavorable winter conditions.
H&E cooling system – a rectangular arrangement for water conservation and plume abatement plus reduced noise emission

Natural draft dry Heller tower with delugable peak cooler cells at the 1400 MW Bursa CCPP

INTEGRATED DRY HELLER AND EVAPORATIVE COOLING TOWERS
A new brand of efficient dry/wet systems has been developed by integrating the dry Heller system and an evaporative cooling tower. The integration can be either in parallel through a combined condenser (H&E – Heller and Evaporative Cooling System) or in series through a water-to-water heat exchanger (SH&E – Series Heller and Evaporative Cooling System). These systems offer great operational flexibility, high availability, much better environmental compatibility than the wet cooling tower, improved summertime heat rejection and remarkably lower investment costs than all dry cooling plants. The yearly water requirement is somewhere in the range of 20-70 percent that of a wet cooling tower. The operational flexibility is revealed via: • The opportunity of economic operation regimes at a wide range of make-up water consumption (i.e. offering easy adoption to the periodical availability of water). • The possibility of operating in dry/wet, but also in all dry mode depending on the ambient temperature and the power plant load conditions.

The selected ratio of the heat rejection by the dry and wet sections is generally based on water conservation targets and on the cost of make-up water. It is worth making a proper analysis, to investigate the optimum combination at different sets of parameters. The project offers great operational flexibility in dividing the head load between the dry and evaporative parts. The ratio can be changed daily or seasonally, depending on water availability, power demand and the ambient conditions. These technologies can also be used to convert existing all wet cooling towers to water conserving ones without changing the existing LP steam turbine. Several arrangement and layout options have been developed for both combined cooling systems (H&E and SH&E), including those that, in addition to water conservation, also support plume abatement and reduced noise emission. Any combination of natural draft and mechanical draft can be achieved for both, the dry Heller System and wet cooling tower sub-systems. With a wide range of applications and options, this kind of dry/wet cooling system offers excellent value for money with reduced sensitivity to water availability and cost. Therefore, it is easy to predict that these systems will have an increasing market share in a more environment conscious future. I

GEA-EGI is able to supply efficient power and process cooling systems for any unit rating or climatic conditions independently of the available quantity of water. The chart herein introduces EGI’s power cooling system variants classified by their year-round water consumption relative to that of wet cooling.

Zoltán Szabó, a retired President & CEO of GEA-EGI is currently an advisor to the company For more information about GEA/EGI, visit www.gdsinternational.com/readerenquiry and enter 1803. Alternatively, visit the Infocentre online at www.gdsinternational.com.

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