ENERGY ALTERNATIVES

Cooling off with steam

Steam-fired chilling technology will create summer demand for

energy from the "Furnace of the city"

By Kris Chari, Technical Editor



"Furnace of the city" was how a recent issue of the Toronto Star

referred to the steam production facilities of the Toronto District

Heating Corporation (TDHC). The corporation's three boiler plants

have a combined capacity of one and a half million pounds per hour

of saturated steam -- the equivalent in electrical terms of around

525 MW, larger than the combined power generation capacity in

Prince Edward Island, Yukon and the North West Territories.

Not many among the thousands who come each day to downtown Toronto

for work or entertainment know much about TDHC's role in the winter

heating of many of the city's best-known buildings. Most people

might think that each of these has its own heating supply: a

totally wrong assumption. Some 15 kilometres of piping, carrying

steam from TDHC's boiler plants, form a distribution network

supplying heat to over a hundred of Toronto's most impressive and

elite buildings which house the city's government, business, arts

and entertainment, health-care and other sectors. Stretching from

Queen's Park (seat of the provincial government) in the north to

the lake-shore in the south, this network covers the government,

financial and business districts of the city core. See Figure I.

TDHC's steam -- supplied at a pressure of 200 psi and reduced at

the customer's end to around 12-15 psi -- brings in the thermal

energy for the buildings' hot water-based heating systems, in place

of the traditional stand-alone water boiler in each building. This

hot water, pumped throughout the building, heats it through a

separate system of radiators. Such district heating systems exist

in other cities as well -- in Montreal and Vancouver, for example -

- serving the city centres, characterized by high building

densities.

As a substitute for a host of stand-alone, individual boiler

installations scattered through the multitude of buildings,

district heating systems bring several operational, economic and

environmental benefits: To mention just a few, improved reliability

through superior operations and system maintenance; flexibility in

choice of fuel used, to optimize the factors of cost, availability

and emissions impact; energy efficiency and cost economies

resulting from a centralized energy infrastructure with a high

throughput; and reduction of adverse impact on the urban

environment.

The highly seasonal nature of the demand for thermal energy is,

however, a major drag on the economic operation of district heating

systems. For TDHC, which generates up to a million pounds of steam

per hour in the peak winter months, summer demand plummets to a

mere 100,000 pounds -- less than ten per cent of the installed

capacity. See Figure II. As well, the economics of their operations

are of direct relevance to their customers, since TDHC is a "not-

for-profit" corporation whose "profits" accrue to the benefit of

the customers. In ten years of operations, TDHC has translated this

into a series of reductions in steam prices.

Cogeneration of electric power suggests itself first as the obvious

solution to the problem. However, if cogeneration were to be the

exclusive route to pursue, the magnitude of capacity available

would call for very significant levels of integration with the

local utility's distribution network, with all the attendant

operational and economic implications. TDHC have sought an outlet

for their spare capacity not primarily in cogeneration, but in the

substantial summer-time energy demand for cooling in the same

downtown structures. This choice is a near-perfect response to the

two basic challenges in finding  added demand for the seasonally

idled capacity: "when needed?" (summer months) and "where needed ?"

(right in their own backyard).

Steam-fired absorption chilling is the means for implementing this

strategy. Developed in Sweden some 70 years ago, this technology

has gained wide acceptance only since the 1970s. Recognizing its

potential as an innovative way to reduce dependence on expensive

imported fuels, Japanese industry focused on updating/improving

this technology. In the five year period ending 1992, about 90 per

cent of Japan's air-conditioning needs were met with sales of over

18,000 absorption chillers (40 tons and larger).

Like the (better-known) compression refrigeration cycle, the

absorption cooling cycle also utilizes the latent heat of

evaporation of a refrigerant to remove the heat from the entering

chilled water. The compression cycle uses a chlorine-based

refrigerant and a compressor to transport the refrigerant vapour to

the condenser. In contrast, the absorption cycle employs water as

the refrigerant, along with an absorbent (lithium bromide) to

absorb the vaporized water. The refrigerant vapour is released from

this solution of lithium bromide by the application of heat, and

then condensed in the condenser. In the chilling system

installations undertaken by TDHC, steam from their boiler plants

will provide the heat for releasing the water vapour from the

lithium bromide solution.

In essence, the process involves four components:

A generator in which the refrigerant (water) vapour is released

from the dilute lithium bromide solution by the application of

heat.

A condenser to condense the vapour (from the generator) into water,

transferring heat from the refrigerant to the cooling water tower.

An evaporator in which the now-liquid refrigerant (from the

condenser) is boiled by distribution over evaporator tubes. The

absorption cycle, which operates in a vacuum, permits the

refrigerant to boil at a lower temperature. The latent heat for its

evaporation is drawn from the entering chilled water, which is

consequently cooled.

An absorber where the water vapour (from the evaporator) is

absorbed by the concentrated lithium bromide (from the generator)

to produce diluted lithium bromide solution.

With this diluted solution pumped back to the generator, the cycle

starts once again. Figure III shows schematically an enhanced

version of the absorption cycle which, while applying these basic

principles, maximizes recovery of the available system energy by

the addition of heat exchangers and a second generator.

Absorption chilling system installations of two categories are

currently under way. Stand-alone systems, at the customer's site,

are dedicated to the cooling needs of the specific building(s) they

serve. The first of these, of capacity 1,000 tons, at the 60-storey

Commerce Court Tower went into operation in the summer of '94.

Similar systems,  totalling 5,650 tons of capacity, at a trio of

Toronto's major hospitals -- The Toronto Hospital, St. Michael's

and Princess Margaret -- are expected to go on stream in time for

the '96 summer. Together, these  installations add up to the

electrical equivalent of around 4.5 MW capacity.

A central plant, producing chilled water (at approx. 50 degrees F)

to be piped to several customers' sites through a separate

distribution network, is the second approach. TDHC's central plant

is currently under construction in the "Railway lands" adjoining

the SkyDome, home of the Toronto Blue Jays. Planned for a final

capacity of 24,000 tons of chilling (nearly 20 MW), the project's

total cost, including a half-kilometre long distribution network,

is estimated at $30 million. Completion of the first phase of the

project will enable delivery, commencing '96 summer, of 3,000 tons

of chilling capacity to the Metro Toronto Convention Centre nearby.

District cooling systems are being promoted as being cost-

competitive, having lower maintenance needs (due to fewer moving

parts), and bringing high system reliability. Beyond these

considerations, they are also seen as being effective in addressing

several public policy concerns:

Ozone depletion by the chlorofluorocarbon (CFC) and the substitute

hydrochlorofluorocarbon (HCFC) refrigerants in electrically driven,

compressor chilling systems.

Global warming by "greenhouse gases" from inefficient burning of

fossil fuels.

Low level atmospheric pollution (acid and particulate matter

emission) due to improper choice of fuels.

Aesthetic impact of countless number of chimneys and cooling towers

on the urban landscape.

How does all this relate to the recent energy efficiency

initiatives in Ontario? At the base of these initiatives lie

concerns about utilization levels of capacities in place,

environmental impact of emissions from the energy industry and 

depletion of fossil fuel resources. Absorption chilling for

powering our cooling needs adds another elegant instrument in

tackling energy inefficiency. A brief comment from Dave Heaslip,

Senior Vice President, Property Development at CIBC Development

Corporation sums it up: "By switching to steam, we provide our

tenants with a comfortable space without negative environmental

impacts, and significantly reduce our use of electricity."