Abstract

Keywords

CCHP technologies

prime movers

thermally activated cooling system

MGT

Stirling engine

fuel cell

internal combustion engine

absorption chiller

adsorption chiller

desiccant dehumidifier

2.1. Introduction

By combining different types of prime movers with different heating and cooling systems, many CCHP systems can be designed hypothetically. According to the previous chapter, the prime movers that can be used in CCHP systems include different types of industrial steam turbines (ST), industrial gas turbines (GT), reciprocating internal combustion engines1 (IC), micro-gas turbines (MGT), micro-steam turbines (MST), different types of Stirling engines (STR) and fuel cells. In addition, the cooling systems that are common in CCHP systems include absorption chillers, adsorption chillers, desiccant dehumidifiers, and ejector cooling systems. Moreover electrical compression chillers are used in parallel with other cooling systems. Other equipment includes different boilers, heaters, heat exchangers, pumps, generators, etc. Because of the differences in operation, price, and environmental effects of prime movers, CCHP systems also operate differently. To help in becoming familiar with the principals of operation of CCHP systems, in the following we present the basic CCHP cycles.

2.2. Basic CCHP Cycles

2.2.1. CCHP Based on Industrial Steam Turbines (ST)

Industrial steam turbines use saturated or superheated pressurized steam to rotate the rotor of the steam turbine. Steam can be produced by burning fossil fuels such as natural gas, and releasing their chemical energy to heat the high-pressure liquid in the boiler tubes (water wall tubes, risers, down-comers, superheaters, and economizers). Steam can also be produced by a heat recovery steam generator (HRSG) placed, for example, at the exhaust of a gas turbine. Some steam turbines also can use the low-pressure steam of some processes to produce power. The thermal energy and potential energy of steam is converted to kinetic energy due to steam expansion in the stationary nozzle buckets (called stators) of the steam turbine; steam jets containing high kinetic energy produce mechanical energy when the rotor rotates due to these jets striking the rotor buckets. This mechanical energy can be used to produce electricity by coupling the steam turbine rotor to a generator. Steam can remain in the steam turbine and produce mechanical energy until its pressure reaches the condenser pressure or downstream pressure of the turbine. Another parameter that restricts the steam residue time in the steam turbine is the steam quality and liquid water content in the steam when it reaches the last stages of the steam turbine. The steam temperature, pressure, and quality are important when it leaves the turbine if it is supposed to be used for cooling and heating purposes in CCHP systems.

Figure 2.1 shows the basic characteristics of steam turbines such as operating pressure, temperature, capacity, and thermal efficiency produced by the Siemens Company [1] in the last century. The steam turbines presented in this figure are simple steam turbines without reheating (from 1900 to 1920), steam turbines with reheating (from 1920 to 1960) and supercritical steam turbines (from 1960).

Industrial steam turbines are categorized based on application, construction, and bucket row type. A diagram of these classifications is given in Figure 2.2.

According to the applications summarized in Figure 2.2, steam turbines can be classified into six types. Saturated steam turbines work with saturated steam; additional superheating equipment is not needed. They can be utilized to produce power from low quality steam, ram pumps, blowers, etc. They may be used in organic Rankine cycles (ORC) as well, because some of the refrigerants become superheated automatically as they enter the turbine in saturated condition. Low-pressure steam turbines make use of the low-pressure steam of processes or other turbines to produce electricity, turn water pumps, operate blowers, etc. The steam inlet pressure of these turbines is usually smaller than 1 MPa. Condensing steam turbines bring the exhaust pressure of the turbine to below the atmospheric pressure. Therefore they are equipped with a condensing system that works under the atmospheric pressure. Sealing of the condenser against entering ambient air is critical. This type of steam turbine is not proper for CCHP purposes, because the exhaust has very low pressure and temperature.

Backpressure steam turbines have relatively high-pressure exhaust steam; therefore the exhaust can be used for running low-pressure steam turbines, condensing steam turbines, or ORC. The exhaust of these turbines can also be used for heating or cooling purposes (CCHP). Vertical steam turbines use low-pressure steam to run compressors, blowers, and pumps in vertical position. Extraction/induction steam turbines have extraction/induction lines in the intermediate stages. The extraction line can be used for CCHP purposes. The induction line also can be used for CCHP purposes, because induction steam can be used for cooling or heating before induction into the turbine.

As discussed above, among different types of steam turbines only the backpressure and extraction/induction steam turbines can be used for CCHP purposes. Figures 2.3A and B show the application of backpressure and extraction/induction steam turbines in CCHP systems.

Industrial steam turbines usually can produce several MWs of electricity; therefore they can be used for large-scale CCHP systems that are especially proper for hospitals, large commercial, or residential complexes [2]. CCHP systems are classified into large scale (greater than 1 MW), small scale (smaller than 1 MW), mini (smaller than 500 kW) and micro (smaller than 20 kW) for [3].

Steam turbines work in a wide range of steam pressures. The steam pressure can reach as high as 3500 psig (241.32 bar (g)) at the inlet and as low as 0.5 psia (0.034 bar (a)) at the exit of the steam turbine [4]. Steam turbines can work with different gaseous, liquid, and solid fuels. They have a very long lifetime (more than 50 years) if operated and maintained properly. Controlling high and low water level is very important to avoid harmful phenomena such as carryover and overheating in the turbine and boiler tubes. Also, chemical treatment of water is extremely important to avoid scale formation and corrosion in the boiler tubes, drum, and steam turbine. In addition steam quality in the inlet and outlet of the turbine should be controlled to avoid erosion of turbine blades. Moreover, combustion should be controlled to keep the boiler tubes clean from outside elements. Every type of deposit on the outside surface of tubes can result in overheating and failure of tubes.

Steam turbines are constructed in sizes from about 100 kW to more than 250 MW. Since steam turbines are designed to provide base loads, they usually work constantly for a long time and steam temperature experiences small changes during operation. The startup time of steam turbines is long; large steam turbines may take 24 hours or more to start up. In Table 2.1, some characteristics of three sizes of steam turbine from TurboSteam, Inc. are compared and presented [4].