Functional Design of the mCCHP-RES System - Design for Micro-Combined Cooling, Heating and Power Systems - page 272

Table 12 Typical overall heat transfer coef cients
Hot fluid U [W/m 2 C] Cold fluid

Heat exchangers Water Water 800 - 1,500

Coolers Organic solvents Water 250 - 750

Heaters Steam Water 1,500 - 4,000

Condensers Aqueous vapor Water 1,000 - 1,500

Vaporizers Steam Aqueous solutions 1,000 - 1,500

Q

A ¼ ð 51 Þ

k LMTD

Q

or A ¼ ð 52 Þ

U D#

where: LMTD logarithmic mean temperature difference between the temperature

of the primary and secondary circuits;
U

—

— Value is the overall heat transfer coef cients (as example, the typical

U-Value are given in Table 12 ).
By choosing the number of plates and their spacing, the heat flow transmitted by

the heat exchanger is easily changed and even optimized to meet a particular
speci cation.

H. Hot water distribution

The residential house may use one of the several types of heating systems. They can
range from blowing hot air through ductwork to piping hot water, through oor.

The most popular types of home heating systems are the following: forced air,
radiant heat, hydronic (hot water base), and steam radiant. Hot water system uses
heated water to heat a space by a combination of radiation and convection. Hot
water in the buffer tank is piped to radiators mounted along walls. The fins increase

the surface area of heat dissipation making the unit more ef cient. Air is distributed

by convection as air rises and is heated by the radiators. Buffer tank can be bivalent
(Fig. 17 ) or trivalent (Fig. 18 ) depending on the number of heat sources.

I. Solar controller

Solar controllers are devices that are manufactured speci cally to control solar

systems. Controller command is generally based on the temperature difference
between solar collector and heat storage. These commands start/stop the circulation
pumps. The controllers will differ in the number and type of inputs as well as the
number and type of outputs. Solar controllers are a convenient and inexpensive way
for controlling commercial solar systems. They come preprogrammed with many of
the common systems already in them and are easy to connect to the overall system.
The units provide excellent data logging capabilities and this data can be accessed
over the Internet.